Honestly, the Metal Sonic costume was the entire reason I started playing Spiral Knights to begin with. The game is great, don't get me wrong, but changing up your Appearance is a huge draw for the game. And honestly, I'd love to see you guys take this a step further!
For example, look at "Sonic R", the old Racing game. In that game, you could unlock a Teddy-bear version of Tails, and a Cybernetic version of Knuckles -- both of those would be great inspiration here! As well, while the Metal Sonic costume looks sick, maybe also add a newer, more pronounced version based on the "New Sonic" designer, with a deeper-blue color, longer quills, and the "SOAP Scorchers" shoes! Or for that matter, you've got E-102 "Gamma", as well as Super Sonic himself.
Another series I could see being cool is "Phantasy Star Online". Specifically HUmar, RAmar, and the Casts/Robots of that game (ie. Elenor, Kireek).
Also, as a personal preference, when using these Costumes, could you automatically change the Eyes/Personal-Color when wearing these items? Just to give them a proper visual (ie. Meal Sonic outfit would have Red eyes, "New Sonic" items would have Green, etc).
How can you say Sonic R, and not mention someone like this one? A themed armor of this guy would be awesome. No one remembers him? But yep, +1 absolutely.