Fan artists love Assassin's Creed. You know why? When you strip away the setting and back story it's just a guy in a white hooded outfit; a guy that could be anybody. It's such an instantly-recognizable outfit that all an artist has to do is paste in the face of their favorite character and it's on like an Assassin's Creed fan art crossover.
No, I couldn't find a Donkey Kong crossover, though that doesn't mean one doesn't exist.
Not all fan artists go the easy route. Some, like the ones I've chosen to highlight here, really made the characters their own. Well, not their own, but two different...you know what? Just look at the damn pictures.
Artist: Gual-Kum (DeviantArt)
No, that's not Alvin. That would be too easy. That's the best Chipmunk. That's Theodore. He'll gut you like a pig, but leave your nuts intact.
Artist: OrcaizerAl (DeviantArt)
OrcaizerAl has a whole series of Pokemon / Assassin's Creed crossover images. Hit up his link to catch em' all—and kill them.
Artist: -D4N13L- designs & stuff
He's the best at what he does, and what he does isn't pretty, at least not until the new game engine arrived.
The Assassin of Zelda
Artist: =uniqueLegend (DeviantArt)
=UniqueLegend's excellent depiction of Link the master assassin has been all over the internet. Folks have even colored it and redistributed it as their own. Sounds like Link needs to make those people a little visit.
Artist: KIRA-THASMO (DevianArt)
Assassin Homer has donuts that pop out of his sleeves with the flick of his wrist. They aren't particularly deadly but mmmm, donuts.
Artist: Anowia (DeviantArt)
There are tons of My Little Pony / Assassin's Creed crossover works out there, but Anowia's stood out as the most elegant and refined, and those are important when you're killing someone.
To be completely honest, I'd play any one of these games in a heartbeat. Don't lie; you would too.