Meanwhile, here I am thinking Roy is actually the most out of place FE character in Sm4sh. His game hasn't been released or remade in the west, he's a semi-clone and was a clone in Melee and he literally only has his Melee fanbase to really care about him since from what I've heard, he's pretty meh in his own game. Mewtwo being cut from Brawl was a bigger deal since he was actually unique. Granted, I see a problem with Corrin's inclusion since he cost extra, was put in purely for promotion of another game and took more time than any clone would. None of those are bad on their own but compare him to Melee Roy who was also put in purely for promotion purposes but he was an easy to put in clone and didn't cost anything extra.
Mind you, I actually like Corrin and Roy (the latter being one of my secondaries) but another issue I have with the promotion bit is that, to me, Smash is a game about video game all stars and characters people have grown to love getting together and fighting each other. You got characters like Sheik and Duck Hunt who've only appeared in one (mainstream) game and yet due to their popularity, they got to stay. I'd like to believe lack of popularity is exactly why Lucas and Wolf got cut going from Brawl to Sm4sh. Roy back in Melee and Corrin for Sm4sh didn't have that. People couldn't be attached to them (not counting Japan with Corrin I believe? Unrelated but assuming the translation team didn't completely change Corrin's personality, how could anyone enjoy the character in FE Fates? I find him/her to be so damn boring.)
Oh and for that post asking for characters from series that first appeared in Smash 64, all I'd want is Ganon from aLttP and both Toon Zelda and Toon Ganondorf. Maybe also Black Shadow from F-ZERO if he takes Falcondorf's moveset.