There's so many "obscure/WTF" picks I want in the game, most of them are retro or just not really popular.
Mach Rider and Star Man are both retro from the NES era, but Pro Wrestling sold a ****ton of units (one of the best-selling actually) compared to Mach Rider. However, the rifle comment made by Sakurai gives Mach Rider hope, plus he's on a dang motorcycle. Ayumi, though I don't know much of, is literally just Nintendo Phoenix Wright in terms of mechanics potential. If MvC3 made him work, Nintendo can make her work.
Black Shadow from F-Zero was made a joke in GX's Story Mode but is a total
chad in every other installment. Zoda's trophy in Smash references Black Shadow being the one who made him powerful, which is only true in the anime and its subsequent spinoff games. It doesn't heighten his chances, but they make it aware of its existence.
Captain Rainbow is an obscure game about obscure games. That's pretty much it, he's the amalgamation of WTF characters that became one when his game didn't sell that well.
Ninten and Giygas, while pretty notable and well-known in the Mother franchise, don't have much going for them outside of it. I don't know how well Mother 1 sold on the Wii U, but I can't guarantee it's as impactful as Mother 2/3. Porky and Claus also share equally high potential for taking a piece of that Smash cake, as Giygas in question would only be using my trademarked
Mewtwo on life support body.
M Rated games are also cool with Bayonetta and Snake being included. Jack Cayman from Madworld and Travis Touchdown from No More Heroes, both on the Wii respectively. However, Jack's SEGA property and Travis is Ubisoft property, and we all know Rayman's first in line.
I could add more but then it'd just be spam.