I know this is a little far out there, seeing as how Sonic and Spiral Knights are two different games and worlds, but heck, I think it'd be fun to beat up some of Eggman's lackeys.
Maybe an undead/incomplete Metal Sonic that becomes more alive/complete as the fight goes on?
I'm not going to lie, this sound really cool. It also sounds like something that would be a DLC level, like OCH. Hope you don't mind if I build upon this:
Of course the DLC would be the same price as OCH. It'd also have the 3 tiers and a special weapon (or maybe a special pet? OOO did say they may make more pets in the future, obviously to correspond with the 2 unused minerals Moonstone and Valestone.)
From the Sonic games way back when they first came out, for me, the most memorable levels were the Green Hill Zones, (obviously) the industrial zone (not sure if that was the actual name, but I hope the readers know what I'm talking about), and the Casino levels. They could make maps based on these levels, going in that order. The Casino obviously wouldn't offer its riches until you beat the end boss. Now, Eggman is okay, but for the Casino level, somehow Metal Sonic seems like a better choice for an end boss.
Here's how the enemies would plan out:
Green Hill Zone-
Beast and Construct (normal theme)
Industrial Zone-
Gremlins and constructs
Casino Zone-
Gremlins, constructs, & one random enemy type which will vary between each playthrough (just to make things interesting)
When you reach the end boss, some Sonic-themed music would definitely seem appropriate. Metal Sonic should have an attack pattern slightly similar to the Collector in the rank mission 2-2 (I think) but also maintain most of his traits from the Sonic games. I think he should be about the same size as your character, maybe a little bigger.
Anyways, once you defeat Metal Sonic, you should get
1. A ton of loot that pours out of slot machines in a room behind Metal Sonic
and 2. A Metal Sonic Set, which will be proudly displayed on a mantel behind a glass case, right before you reach the lift. It will be dark at first, but then once you hit a button, a light will illuminate it and show you your prize.
People are going to hate me for this, but for the sake of keeping this accessory's value up a little bit, the set will be bound to your character so it wont be marketable or tradeable.
As far as a unique weapon, I have no idea. Maybe a bomb that inflicts pure elemental or shadow damage that also has a high chance of inflicting poison? For a unique pet, that's obvious. You could pick up a sprite based off of Chao. The reason I say this is because we already have a Sonic and Tails accessory, and since Chao is blue, he could be the pet that winds up using the Moonstone to level up.
Any thoughts?