I was happy to see Powdered Toast Man in here. I know, I know, but I still have fond memories of Ren and Stimpy as a breath of fresh air in early 1990s cartoons, and the muscular mascot of a highly improbable instant breakfast still makes me chuckle.
Here's what I'm hoping will be included in the game:
El Tigre
Rocko (guess that's just about necessary)
Jenny/XJ-9 (from My Life as a Teenage Robot)
The Flesh (he's super strong... and super naked!)
Vendetta (from Making Fiends. She makes fiends!)
Jimmy Neutron (or maybe Carl, as a joke character)
The Angry Beavers (fighting in tandem, natch)
Aang (maybe Sokka, I dunno. Someone from Avatar)
Garfield...? (I mean, hey, he's Nick's property now, and he's orange)
Little Bear (okay, that's just me wanting some bear representation)
some damn real voices this time, because Game Mill's Nick racing games didn't have them
As for bosses, maybe combine villains from two of the series. Plankton and Silver Shell for that double Sheldoned-in goodness! Shredder and
Vexus as a samurai... pharaoh... Borg queen thing! Also, there needs to be some of Nick's game shows as levels, particularly Guts and Legends of the Hidden Temple.