I'm pretty satisfied with what we got for 3rd partys/new series right now and with Sakurai saying this will probably be the pinnacle of smash games regarding the number of fighters, I'd like them to fill the hypothetically 10 remainig slots by "filling the gaps":
01. Dixie Kong
02. Bandana Dee
03. Waluigi
04. Zelda Rep (Impa, Tingle or Skull Kid)
05. Geno
06. Isaac
07. Rex (and Pyra)
08. 2nd Sonic Rep (Tails, Robotnik or Geno)
09. Sceptile
10. Ashley
Of course, this is pretty much wishful thinking, as most of these won't sell as much as more new 3rd party franchises... I have a little hope for Dixie and Impa as Echo fighters, but right now I wouldn't bet on it.
So to be more realistic:
01. Amaterasu
02. 2B
03. Sora
04. Lara Croft
05. Lloyd Irving
06. Jill Valentine/Leon Kennedy
07. Spyro
08. Crash
09. Steve
10. Rayman
11. Dante (because there ist still one open slot in the fighters pass)
However, I'm pretty open for any character joining the already fantastic roster, the only characters I didn't want, were non-video game characters, which Sakurai already shut down, so... bring'em on.