1. Paper Mario (Paper Mario)
It would offer an amazing character with a very interesting moveset, very different from the original Mario. Using his party from his games would be instant fun. Also, a lot of stages from the franchise would work. I'm also 100% certain he will be in.
2. Ridley (Metroid)
Nobody doesn't want him. He's simply perfect, he deserves a spot.
3. Yoshimitsu (Soul Calibur / Tekken)
Yep. Good old Yoshimitsu. Why him? Because he's the only Namco/Bandai character who represents two franchises at the same time, and he's also the one that fits the most in the Smash Bros. universe. Anyway, he would mostly incarnate his SC character more than Tekken:
http://www.youtube.com/watch?v=4ncwH3Oj7JU
Trust me, he totally works. It'll never happen, but it would totally work. Also, we have our mind blown character after Snake. Just imagine the deathcopter, the poison breath, the vaulting flea, the iridian stance... He's perfect.
4. King K. Rool (Donkey Kong Country)
Same as Ridley, he deserves the spot and he would work. Why the hell not.
5. Professor Layton (Professor Layton)
He's awesome, simple as that. I'm not sure how he would work moveset-like, but I don't care, he's one of the most important DS characters, they can work something out.
6. Simon Belmont (Castlevania)
The second Konami rep and probably the last one if you ask me. It ties with Nintendo's history very well, it's both a third party and a retro character, I'm mostly interested in his possible stage. I think he deserves the spot.
7. Palutena (Kid Icarus)
Uprising is still the best 3DS game and I'm pretty sure that Pit won't be the only character from the franchise in the next Smash Bros. So Palutena is the obvious choice, even if I would like Medusa too. Anyway, both are a stretch, so one is enough for me.
8. Soldier (Battalion Wars)
I'm somehow interested in this "character", which would be basically a gun fighter even more than Snake. The Wars franchise anyway deserves a spot, it doesn't matter if from the Battalion spin-off or from the Advance series.
9. Bomberman (Bomberman)
Always loved this guy, and now that Hudson Soft fell it should be easier to get the permission (right?).
10. Lord Ghirahim (The Legend Of Zelda)
He's probably one of the most controversial Nintendo characters after Birdo, and it's easy to understand why. It doesn't surprise me that sometimes he gets a lot of hate, but he is undeniably one of the best developed villain of the franchise. The fact that he's probably not going to appear again loweres his chances, but that's ok.
Obviously I kept out of the list the impossible characters (even if you could say that some of these are actually almost impossible). In that case, Banjo-Kazooie or Conker would top any reasonable and interesting list of this sort. I also tried to put some original character in it just to make some new idea.
Other characters that I would love and that are out of the list just for a little are: Rayman, Bowser Jr., PacMan, Phoenix Wright, Pico from F-Zero, Genesect, Mother Brain, Tom Nook, and a lot more...