The only two newcomers I desperately want since Mega Man and Rosalina have made the cut, are Professor Layton and Reggie.
The former is a no brainer, if you ask me. One of the most popular DS franchises out there, and if you look at the "Nintendo Characters Hub" on Club Nintendo, Layton is there. That, and it wouldn't be that hard to make a moveset for him. Layton has had his share of fights, and has taken care of threatening characters in all manner of ways.
Reggie may be a bit of a pipe dream, and I know a lot of people may not like the idea of a real person being in Smash, but I think he is iconic enough. Plus, how cool would an "E3 Stage" arena be?
I did sign the Reggie petition though, and I am a bit hopeful, seeing as it was acknowledged by him on multiple occasions.
Other than those two, I expect/sort of want Dillon and Lady Palutena. I haven't really played their respective games, but from what I have played of the former and seen of the latter, they seem like really endearing characters!