I think the next character reveal will be the "wtf" addition. We've always had one of those in every game (Jigglypuff in 64, G&W in Melee, Rob in Brawl and Wii Fit Trainer in SSB4) so i don't see it being different this time.
Retro/WTF sometimes go hand in hand (G+W and ROB come to mind), so I am banking on that happening this game. I am torn between Hikari and Donbe(Shin Onigashima), Lip (Panel De Pon), Ayumi(Famicom detective), Mach Rider (Mach Rider) or Prince Sable(I won't even try to spell that one). With Takumaru still a trophy, I see those 5 as the logical choices, rated in order of who I want the most. Though the most likely one would probably be Mach Rider or Ayumi, as Sakurai has stated he wanted them in smash in the past. Lip also has a great chance due to her item being long in the series, and really is the only item in this game tied to a game series with no character playable.
Mach Rider removed from Melee due to not sure how to make a moveset work for them, but with Duck hunt taking from all light gun games, they(since we aren't sure if they are male or female), could take from a few classic Nintendo Racing games. Heck Maybe even having them use Excite bike somehow, like a side B sending out a mini biker to attack.
Ayumi was removed as Sakurai was worried that it was a Japanese only game, but with the world more connected then before, this could not be as big of an issue. Plus with Duck Hunt a fighting style might be better for him to design now then back in Melee.
Lip could represent the Puzzle game genre as a whole, as we don't have that in this game. She will use her Lip's stick item of course, and her smash attacks could add flowers to the heads, but she could also drop blocks, use tetris stuff, or other flower like abilities.
Though I myself Want Hikari and Donbe the most, due to they look cool, the game is unique, and could have a cool fighting style, I know their chances are slim. Still really want them.