Tbh I'm not sure why they chose Bayonetta over the Inklings. I mean I love Bayonetta but Inklings just seem like they'd fit better. Not only are they Nintendo characters, but Splatoon is kinda like the rival to Smash right now.
If they don't get in the next Smash I honestly would not know what to do with myself.
They're probably saving them so they have "big" characters to get people interested in the next game. If the newcomers for the next game with FE guys and like Krystal or someone people probably wouldn't be as hyped, but eh, maybe I'm wrong, but that's just my guess.