Shit, I totally forgot this one that I loved the idea of during Brawl's character announcements: Masks Link
Take Young Link from Majora's Mask, and have him alternate between his three forms: Deku, Goron, and Zora Link. (Similar to how the Pokemon Trainer switches between the three pokemon). The final smash would have to be putting on the Fierce Diety mask and being a badass that way.
ummn.. maybe that "Awakened" Mewtwo
Magno from Kid Ikarus Uprising, he's very likely to be a playable character, I hope so
Palutena, Ades (?¿)
I like so much the idea of Young Link with the masks and that final smash of the Fierce Diety
Zero from MegaMan Zero
maybe Zoroark will be a NewComer
obviously Shadow from Sonic!!
I see Reggie as ridiculous... Some people may disagree, but if Reggie can be put in, then Iwata and Miyamoto are also realistic choices... At that point its just Nintendo Smash Bros. I see the humour in adding Reggie to SSB, I can can imagine "My Body Is Ready" attacks and what not, but yeah, I'd rather him not being put in.
As for realistic choices...
Sora (Unlikely I feel, but definitely possible seeing hes on Nintendo platforms and I feel KH has been good for Nintendo)
Ghirahim (Just cuz, ha ha, seems cool enough)
Layton (Again, done a lot for Nintendo, deserves to be in it)
Phoenix Wright (Again-Again, done a lot for Nintendo, deserves to be in it)
Sonic (I knows hes been in it before, but there isnt a guarantee he'll be in the next one and I feel he should and deserves to)
Shulk (Comes from a very good game and Monolith seem to be enjoying a closer relationship with Nintendo, so you never know..., I'd love him in!)
MASSIF BROS! (Just kidding, but I love those guys!)
Ridley and Dark Samus would be awesome. I know Ridley is huge, but why not scale him down? After all, in Brawl he was significantly smaller than his prime counterpart. Dark Samus would be awesome as well. She wouldn't have to be a clone either . I doubt either will be playable though since they haven't really cared to expand the Metroid Universe in smash, albeit most Metroid characters would be unreasonable to add.
Now for the hopes and dreams character: Conker. He'd be so hilarious, but of course this won't happen XD
Reggie would be great as an inside joke between Reggie fans and Nintendo, but the casuals won't get the reference. I signed the petition for the lulz anyway. My body is definitely ready.
Of course, Miis (that you could customize) would be great, but Sakurai seems pretty stubborn about them not being suitable (despite the fact that they would allow for a lot of diverse, original movesets), which I think is Nintendo's worst developer decision since Miyamoto destroyed the Paper Mario formula.