How about enemies that have very specific weaknesses, and are very hard (but not impossible) to fight otherwise?
Right now you can destroy anything with the right weapon damage type. Since there are 3 types of damage, and you can have 4 weapon slots at most, you could be amazingly effective against anything the game throws at you by yourself. You have no reason to work with other people since you'll get all the materials and potential item drops. Yes, I know how much more fun it is to play with groups of friends, but if you have little incentive to work with other people, how do you find these friends in the first place?
I think it would be interesting for the majority of enemies to be killable normally, but be much more dangerous unless you have something they're weak to. The variety of enemies would mean that you couldn't just bring 4 weapons and be almost as effective as a coordinated team. Why not add some niches that don't HAVE to be filled, but would greatly help the team if they did?
Examples-
Plate armor should increase shield health. Because.
Lumbers:
Unless their smash attack hits a shield, it will create a large explosion. In tier 3, some projectiles (thorns?) also spew out in random directions. This makes it a lot harder to just dodge around their swings and stab them in the back. A player with a heavier shield will be able to lure the lumbers and tank the hits, while people with lighter/offensive shields (I'm pretty sure barbarous thorn shield can't stand a single hit in tier 3 from what I've seen, swiftstrike is definitely screwed) will take some damage fighting them. This creates more use for the "guardian" playstyle- you can tank lumber hits to save your team instead of just making sure that everyone moves slightly to the left/right.
Silkwings:
Natural resistance (not immunity) to swords. Seriously, how the hell are we hitting them with hip-level horizontal swings? :L Projectiles and bombs should do regular damage to them. Obviously, makes sword usage more difficult, but I'm finding that swords are way too powerful compared to the other two weapons.
Jellies:
Unless poisoned, hitting a jelly will spawn a mini jelly (or three.). Encourages use of support/poisoning weapons that otherwise see very little use.
zombies:
Have an very weak aura of whatever element they are around them unless stunned. (most of) Firestorm citadel is just stupidly easy because the main enemy is a slow moving, slow-hitting enemy that telegraphs its attacks in a very obvious manner. This would make it harder to get out of the most "dangerous" situations by just walking through a crowd and shield bumping. Stun weapons are quite underused at the moment too.
Trojans:
Similar to lumbers in that you need to tank their swing in order to avoid bad things. Instead of an explosion, how about spawning another random small fiend? If you keep running away, you'll suddenly find devilites and grievers chasing after you. This again adds more viability to guardian's heavy armor/shield and turns trojans into fitting "miniboss" monsters.
Gremlins:
Will walk towards placed bombs out of curiosity. Also they get pulsars.
I totally agree that all monsters besides devilite in the game need to be buffed. I have felt this for a while now sense I’ve been playing spiral knights for so long.
The only problem i see with one of your buffs is the trojan. I would rather if its smack attack doesn't hit a shield; it launches a wave of a cut in the direction it smashed. I think spawning fiends will make it just too crazy to defeat and it would cause lag cause of so much monsters in one place.
I also agree that the elements in the game needs to be more emphasized where they have a specific role against specific monsters. Weapons should be this way where having the right weapon, makes the monster very easy to beat and having the wrong one will make it harder to kill. I don’t agree that some of the current monsters we have now should become mini bosses. I think mini bosses should be new created monsters so that we don’t have the same thing they did in shadow lairs (where they took our current bosses we have now and just increased their difficulty and gave them a new skin). That would just destroy what it means to have a new challenger when the new monster you about to face is base on another monster with the same look, same skill, and just increased the difficulty by making it harder for the player to maneuver (like affecting the field and adding fire tiles), and giving the monster more projectiles (like the tier 3 knights). For me a new monster is a new design, new attack pattern, everything about it should not be based on existing monsters. The experienced players may have some knowledge of how it attacks an behave, but not everything about it should be copy and paste.
I like your ideas, i think you can expand more on it though. Keep at it!