Here's some more mechanics that I think would be fun:

A four star Swiftstep Buckler, trading more defense to add MSI low to what already exists.

Overload mode, because people are bored and powerful. Created in arcade levels as a wave of core-energy or whatever rotates around, adding the option of Overload mode to a single gate at a time. Rotates the opposite direction as gates. Cannot, therefore, be used to sit back and farm FSC with little effort.
Doubles enemies' health, damage, status resistances, status strength, etc. Removes caps and diminishing returns from boosts on knights, so you can use double max ASI or walk through traps with hardly a scratch. Probably should increase prizes somewhat, but not too much.

Marshmallow sword, doing damage heals the user IF the user is on fire. Not great damage. Because marshmallows become edible when toasted.

Mirror shield, that has less than stellar defenses, but reflects projectiles. Makes projectiles 'friendly' of course, damaging enemies instead of allies. Highest star version can even reflect FSC wheels.

Mind control pickup, thrown like a vial, that makes the affected enemy attack and be attacked by other enemies. Wears off after a while. May or may not work on bosses.

A shield that, when instructed to shield bash, blinks the user forward instead. Higher star versions could create a shadowy trail, which inflicts a status or damage.

Sticky bombs, covered in jelly. Swung like a short range sword, enemies that are hit explode after a delay. Explosions would need to be weaker than normal bombs, to account for how fast bombs could be stuck to enemies. Affected by bomb boosts, not sword boosts. Charge attack could be that mythical thrown bomb.

Dragons: Chromalisks with Drakon wings, that move like Gorgos and breathe fire like Zombies. Appears in place of a regular chromalisk.

A bomb that teleports the user back to the center of the blast whenever it explodes.

Weapons that gain boosts based on how many attacks have been successful in a row without missing.

A level in which zombies revive UNLESS they're in range of a totem. Obviously, the totem will have a divine color scheme, and the rest of the level would be something scary. A graveyard, maybe.

Gongs, held over one's head like a pot, that you can use to smash and stun things with. Not thrown, smashed with.

A trinket or armor that cures another party member's curse affliction when the user becomes cursed. If more than one party member is cursed, the curse with the longest remaining time is cured. Gran Faust spam, it'll be great.

Devilites could drop office supplies on death, that knights can pick up and use. Similar to vials.

Trap chambers added to the clockworks, similar to that one mission near the beginning.

A gun that takes the antigua's idea of "not great damage, but lots of bullets" even further. Would be great for breaking blocks and cloaks, and getting aggro. Maybe two antiguas? In one hand, if course, or it'd be dual wielding. Can't have that.

Do we have attack speed orbs? If not, let's throw those on the list. Also, status resistance orbs. Unless defense orbs cover that...

A ball and chain sword, where swinging the handle makes the spiked part jump towards (and then past) the user. How fast/far it jumps and how much damage it does will depend on how far the user is. After a certain distance, the spiked part bounces along to follow, keeping at maximum distance. Maximum distance would be somewhere around the range of a Nitronome. When swapping to the weapon, the spiked part begins at the knight's feet. If the spiked part gets caught on a wall or gap, after stretching a bit past the following distance, it will fly back towards the user as if they had swung the handle. It will pass through walls and gaps until it passes the user, after which it won't.

Boomerang, a sword that is thrown and comes back. Swap to another weapon while it's out to keep doing damage as it flies back through the crowd to you. Only passes through enemies (and walls) on the return trip. Having it equipped while it's out flying around is the same as not having a weapon equipped.

Pill vials. Heal your friends. Because everything's better as a vial.

Amplifier rings, that increase the damage and/or range of bullets that go through them. Either found as part of a level or deployed from a pickup. Shooting through three rings and hitting a player wielding a troika line weapon will grant that player a chance to inflict shock for the rest of the level, or until they start using a different weapon. c:

A weapon, pickup, sprite ability, something that creates a totem or block from nowhere, for use in blocking stuff. (But not weighing down switches)

Randomly scatter some dead NPC knight through FSC, it's a bit dull to have the exact same scenery every time. Blocking one or the other options at the fork in the third level might also be fun, make some people try the other side from time to time.

Gremlin pyros' bullet launching attack should do more damage and knockback after a certain range.

A gun with a short range, relatively slow speed, and knockback. Basically, a sword that's affected by gun bonuses, not sword bonuses.

New undead monsters: the zombified gunslinger priests from the gunslinger gear descriptions. Zombies with hats and guns, what could go wrong?

Cautery sword, or maybe some other weapon, cures the user's status effects after defeating a monster with it?

A pickup that signals the same people who run the artillery strike to send in a mini arsenal station, so gear can be changed. Does not drop in boss missions. Not super common, just an occasional backup if you rush into a candlestick keep or something without changing from your jelly stomping loadout.

A weapon, probably similar to autoguns, that gains boosts based on what percentage of the charging time the user was stationary for. Would have slightly better stats after a complete standstill, and slightly poorer stats after having been moving the whole time.

Frankenpuppies, gun puppy trojan mixes. Shoots bullets while relentlessly trudging towards you, and laughs at your puny blitz needles. Never stops facing its target, or holds a monster gate closed. Gotta use teamwork. (or a shivermist buster.)

Leaking vials. Trades most of its range and explosion radius to leave a trail of status effect in its wake. If put in a nearby respawner, could potentially let the above monster hold monster gates closed.

Sleet. Absolute(ly) zero traction. Presumably great for slide-y puzzles, if you limit diagonal movement.

If there are going to be any more battle sprites, before you announce what they are, add the recipes for [adjective] motes, dust, stones, and stars a week or so before the announcement, so people can speculate on what sprite would use a(n) [adjective] [food type]? Might be fun.

Laser sights. Aiming at an enemy causes damage or statuses, because lasers. Also, I suppose it would have the side effect of showing exactly what you'd hit if you were to shoot. Doesn't do spectacular damage, either inherent to certain guns or applied temporarily with a pickup.

Windefinite, an upgrade to the Winmillion. Does not scale up in stratum six like other five star weapons, but projectiles now retrieve items that can be held from afar. Totems, minerals, etc.

Trinket apocalypse! Possible effects include, but are not limited to: Move speed, knockback resistance, luck at finding certain pickups, flaming trails, sleeping at random, flammability, invisibility when standing still, +25% vitapod effectiveness, increased vision range, the ability to see marks on the ground denoting where enemies are set to spawn, a constant draining of health in exchange for a fistful of rewards at the end of the level, extra pill/vial stacking skills, and the ability to use weapon slots to hold even more trinkets.

Here's yet more mechanics that I think would be fun:

A level completely full of blocks. The elevator is next to the start, but behind some unbreakable blocks. Monster blocks abound. No ghost blocks except one at the far end of the path behind a party button so that everyone must participate. Unbreakable blocks line edges of the level, going all the way back to unblock the elevator when evaporated. Several puzzles involving setting up blocks the right way so treasure boxes will be exposed after the ghost block goes off. A couple completely random monsters will spawn in the way back, to imitate a last few monster blocks having opened as the player passes by. Possibly exclusive to elevators marked with a '?'. Which leads into...

'?' Stratums. Instead of a monster or status theme, all elevators in these stratums are marked with a '?' and lead to a different level every time. Marked on the gate map as simply leading to '?', rather than how current '?' elevators are displayed. Treasure vaults and graveyards in this stratum would have nowhere near as high a chance as normal, since they're still only two levels, but the number of other options has increased.

The moooooon! 
Reason: scientific exploration, checking for new mineral flavo- I mean, colors.
Gameplay Gimmick: dashing has doubled range, dramatically decreased cooldown, grants invulnerability to traps, can be initiated while charging bombs, and no longer stops you from attacking. To balance this, MSI of all knights not in mid-dash is dramatically decreased. Magnus lines, and only Magnus lines, have enough force to redirect dashes.
Mode of transportation: the Spiral Order can get knights up to the moon by building a shuttle powered by the power core in the lab.
Result: A new mineral color, unlike any other color or combination of colors! That way another batch of three sprites would fit nicely, instead of only having two. Originally found in the moon's core, to provide the same amplification effect as Cradle's, moved to and now appearing on Cradle.

A weapon that does nothing but generate a vial held above the user's head whenever you swing it. Vials may either be randomized, or organized into different versions of the weapon, like alchemers. Organized curse vials would be a no, but might work in a randomized one. Sleep also wouldn't appear. Not effected by any boosts other than perhaps a swiftstrike buckler.

A weapon that, when used once, drops a marker. Every time after that, it drops another marker and teleports you back to the previous marker, destroying it. Small blast on arriving, so you can still make progress if it's your only weapon. Does not qualify as a weapon in the Hall of Heroes.

A alternative branch to an existing gun whose bullets, after traveling a portion of their distance, tunnel into another dimension and sit there until the next bullet is fired, at which point they tunnel back and travel the rest of the way. If the next bullet had already been fired by the time the reach halfway, they don't tunnel and act normally instead. Possible charge attack gives the next x extra attack speed, and makes them not trigger other bullets' reappearance. Meaning, when you fire x+1 bullets, you get a whole blitz of bullets all reappearing at once. A good number for x would be somewhere around one clip of bullets. Gun should glow when charged, and glow brighter when all charge bullets have been fired and the next shot will set the whole thing off.

A pickup that, when used, moves the user into a slightly different version of the level: one where the rest of the party doesn't exist. The user gets their own copies of each enemy to fight, unaffected by anything the rest of the party does, their max health lowered to what it would be while solo. Killing one of these enemies kills its counterpart back with the rest of the party. Meanwhile, the rest of the party views the user and their set of enemies as ghosts. If the rest of the party kills an enemy on their side, the user sees that enemy as marked with a blue X, but otherwise perfectly fine. These effects all wear off after a short time, maybe 20 seconds? All enemies defeated will remain defeated, and the user will not be required for any party button they'd already pushed while solo. (Don't you hate it when your party alerts rocket puppies, spams high knockback weapons at random, or simply won't get on the button?)

Invisibility orbs. Does not break on attack or damage. Only drops on normal difficulty.

Gremlin Reanimator, a Mender gone mad who revives EVERYTHING in sight, granting shadow resistance in addition to extra life. Creates Bombies when there's nothing else to do, up to a cap of probably four Bombies alive at a time. After that, maybe even goes to work healing as well, but that's really low on his priorities. Mainly seen alongside zombies or other gremlins, but works on any monster family.

Cutters could be usable without breaking cloaks. Ghost blades would still be visible to other knights. Negating knockback done while cloaked would be awesome, but possibly a bit too much.

A pickup or trinket that shows the user the an arrow or line indicating the path to the elevator, without going through walls and stuff. Would not be visible to party members.

Low chance of surprise kleptolisks, anywhere, just to keep people on their toes. This includes Moorcroft, Emberlight, and all terminals.
Kleptolisks appearing this way will not spawn close enough to the entrance to become aware of the player before they move.

Here's some sprite ultimates I think could be made less one sided:

Swap around disintegration ray and dazzling ray so that they do extra damage vs everything, and strip away defenses from fiend/undead.

Instead of trying to hurt things, which they're clearly not good at, the souls created by haunted haze should heal the user.

a) Can't a damage type be used against more than one enemy?
b) They can't rotate while firing. Wait until they fire, Archeron charge, done. Most if not all mist bombs help too. I think they might drop thier shields when inflicted with anything, like gremlin demos drop bombs.