Andy - I rather doubt you'll get Telltale to answer this question (though who knows, I could be wrong). So this thread will probably continue to degenerate into who we forum users would like to see. A degeneration that I shall embrace immediately!
Mario would be neat in theory, but in practice he'd probably be a lousy character because he (quite intentionally) has no personality. He's supposed to be generic enough that you can put yourself into his shoes.
I'd like to see Sonic, but I'd want it to be the old-school 90's Sonic, not the post-Sonic Adventure incarnation. Admittedly, he probably wouldn't be as good a character as the ones we have already, since Sonic isn't quite as mentally unbalanced, so I guess this is just the nostalgia/fanboyism talking...
I'd say The Scout would be good, but I guess that's a rather obvious idea since we already have The Heavy. Also, some people are probably tired of The Scout's chatter by now.
Um, let's see, who else... Leo from VG Cats? (I slightly prefer Aeris as a character, but I think Leo would make more amusing chatter...)
Also, he's not a video game-related character, so he wouldn't belong, but so long as we're fantasizing... I think Davan MacIntire from Something*Positive would be great
. S*P has some of the best dialogue I've ever seen, hands down.