So basically, if you got characters, fill 'em up? Why expand a finished franchise such as Mother or a over-representated franchise such as StarFox when other franchises, that are relevant, can be expanded/introduced? Such Metroid, Donkey Kong, Fire Emblem, F-Zero, Kid Icarus, Golden Sun, Xenoblade, Super Punch Out, Advanced Wars, Sin and Punishment, Shin Onigashima, etc etc etc. what I'm trying to say is why bolster a well-representated series when other series have yet to be well-representated or have no representation? "Oh, Metroid only has Samus so let's throw in Ninten!"
The series I've listed are all active or have received attention (f-zero) in some form. Hence why I think pulling characters from there, to fill a roster spot, is much more beneficial than diving into finished/dead series that have been already represented. Unless Mother/EB will receive a reboot or a port, I'll stand by that Mother/Earthbound has already been represented well enough in the Smash Universe.