Here's some mechanics I'd like to see:
A gun that jumps to hit two enemies, similar to those quills, at the cost of doing around 55%-65% damage.
A gun that, on hitting an enemy, queues that enemy to take damage as soon as a different enemy is hit. Should deal a bit of extra damage to reward the complicated aiming, maybe 10%?
A gun that picks up any loot it shoots over. Attack pattern should be at least similar to a Pulsar with a clip of one, so it can pick up a whole lump of treasure without being spam-able.
A bomb that swaps the user's health percentage or status afflictions with whatever's closest to the center of the blast. Dramatic, but potentially harmful. Looong fuse.
A mist bomb that inflicts enemies with any status infliction they already have.
An ENORMOUS Troika sword that does, like, 200% damage at the expense of slowing down turning speed.
Monsters that are flat out immune to everything but fire damage. Needless to say, they have more treasure than health, and can't hold monster gates closed unless a fire pot is nearby.
Monsters that deliberately inflict more move speed boost than anyone could know what to do with. Make sure to throw some environmental hazards into any level they inhabit.
Bonus enemies that escape and/or disappear quickly after being first hit, but stay still until then, don't attack, and carry plenty of treasure.
Something that grants the ability to change direction mid-dash, via a smaller dash.
Treasure caches in rooms held closed by switches only alchemers or difficult vial throws can reach.
Bouncy plates that let vial and pot throws bounce through convoluted passageways towards switches, and gunfire bounce around arenas in fun ways.
Special treasure blocks that only drop treasure if destroyed simultaneously.
A status effect that removes enemies' invulnerability to environmental damage.
The ability to detonate one's vitapod to spray glittery red "heal goo" on allies, based on how full the extra health granted by the vitapod was.
More forks in paths that give a choice between two arenas, similar to what's in FSC.
A weapon that has a higher percentage of its damage piled onto the last swing/shot of the combo than usual.
Has been suggested before, but underwater levels, featuring debuffs for attack speed, sword and gun damage, and movement speed, along with buffs for bomb radius and damage, plus fire immunity for everyone underwater. Gremlin forcefield generated air pockets can be placed as need be, so sword and gun users can get all their stuff back in some areas. I think knights would be able to breathe underwater fine.
A flamethrower that does minuscule damage at high speeds, for interrupting things.
Buff the Iron Slug so that it's INCAPABLE OF RUNNING OUT OF AMMO. The lack of mobility would be a reasonable limit on how many shots knights can safely shoot, right? Right?
Give Basil the ability to communicate with the Auction House, mainly for price checking reasons. Being able to post auctions would be debatable.
Install old, rusty forges at the start of each boss level, that only function in return for all your vials, barriers, and vitapods. 'Cause, you know, people can functionally do this already.
De-instance the coins on the ground in treasure vaults, since it's a pain to collect them all on both paths. Alts won't be dead at this point to, since it's the start of a new level.
Sprites should be able to digest the materials from Mount Krampus, to deal with all the extra giftwrap.
In Lockdown, it would be cool if recons and guardians had some way to access heal pads as quick as strikers seem to be able to. Maybe strikers could let other classes grapple onto them and waterski behind them. Maybe a bomb with a short fuse that deals BaB style knockback to the user. Maybe teleports back to base for recon, and increased self-healing for the shield. Maybe I should just get gud. Who knows.
Here's some stuff that I think would be pretty:
Accessories visually being knocked off knights' armor as damage is taken, without actually deleting the accessories off the gear of course.
Enemies should give each other high fives, sit down, or just go to sleep whenever all knights present are dead.
Or, if the knights are wearing edible armor, like jelly and frosted helms, nibble on them.
If knights have mustaches equipped, twiddling them should be added to their idle animations.
Strangers and enemies should be vulnerable to snowballs.
A stack of books, reskinned from a Snarby bomb during a Black Kat event, that explodes into a barrage of paper that inflicts paper-cuts AND cures unliteracy.
A shotgun, reskinned from a Snarby sword, with a bayonet on it to explain why it's piercing. Should still be categorized as a sword, not a gun.
It would be complicated to get it to look right, but cutters reskinned to nunchucks or punching weapons would probably make a lot of people happy.
Feel free to respond to only a couple of things, rather than the whole list.
something that renders us invisible similar to the chromalisks would be cool