These are just a few suggestion for extra danger rooms or possibly new tiers of bosses

Mini Bosses:
These would be around the same health as the Giant spike jelly x 2, or if tokens for mini bosses were introduced to make Danger rooms more interesting to play, could have more. less than a boss of the same tier however, these could also include bosses from the prestige mission like the ice trojan, without the boss enviornment however, but this could make them too easy since it's the environment that gives the difficulty.

Cerberus - 3 headed dog, could be easily themed to any status damage or a normal version - Hard or Lower difficulty shared health pool, elite each head has it's own health pool, destroy all three to destroy *This has been suggested elsewhere I believe*

Giant Green Jelly / Tortodone / other unreleased big monster - These were in the beta, never made it to live or removed, place them back as mini bosses in dangerrooms in my opinion

Gremlin bosses - similar to the one in the opening levels or Seerus from the expansion, Just a tougher Gremlin with a more potent ability from a thwacker, fireblazer etc.

Snarby pup - similar abilities to snarby without needing to stun.

Mind controlled / evil / insane Spiral Knight - one of the big beefy lads, has a random load out of weapons from the same tier, could also use PvP kits for increased fun.

Minotaur - We've all played this game, sing along if you know the words, OH NO! He's going to Charge! Quick! Hope out of the Way! Now he's stunned cause he hit his head against the Walllllllll!!!!!! in e minor.

Swarm Keeper - Gremlin or Knight or other clock works creature that loves Swarms! has on a box/bag that he opens up to release them, this is also the time to attack, as he shakes his coat when they stop coming out, could also fall over if attacked too much when shielded. ALTERNATIVELY give him a coat and rename - The Flasher!

Big brother . . . Devilite - a School yard bully devilite, attempts to give your team wedgies and spit wads, can bear hug, slam, and fart at you.
. . . . Ok I might be some what kidding with this one

Mothra! - pretty much what's on the label, giant moth creature, I hate these guys . . . . but a big one would be fun to fight, however different mechanics, slower mover, and cause poison / stun effects on wing flaps *This is totally not ripped off from pokemon, don't care what you say >.>*

^ as above most enemies in the clockworks could have a Giant version - Giant Zombie, Gun puppy *oh wait roarmulus* Lumber mecha knight, so I won't mention any more giant versions as I'm pretty sure you can all see where that would go.

Tank-a-saurus-rex - Similar to the Tortodone but a gun puppy tank, abilities could be quite similar as would ram and fire missles - perhaps too similar but I'll list it anyway

Scorpion Scuttlebot - Before you argue this is another giant creature, it also has a scorpion tail. so neeeeeeeee. Tail can inflict status, with a sting or whip attack, the usual spit mouth attack should also be included. perhaps a defensive curl whilst it summons a horde could be include if wished to phase it for an actual boss tier.

These are just things that could be added to danger rooms or perhaps as new big monsters like the trojans as I do enjoy fighting those as tedious as they can get.

Tokens:
Mini Bosses such and big enemies could have a chance to drop a new token line, to give some extra incentive to hunt them down.
Could add in new weapon lines or materials this way for more content added in later.