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!)