For me it's just after asking for Metroid characters for ages and finally get them in the form of Ridley and Dark Samus, I'm satisfied. I feel I would be insatiable if I just kept pushing for whatever Metroid flavor of the month character to the same extent, even if it's my favorite series. That and there's other series I feel could use more characters now. Fire Emblem threw away the idea of "balancing series by number of reps" long ago, so Metroid getting another character wouldn't be egregious or anything, but it's just no longer a top priority of mine. I'd be fine with Raven Beak though, and Tifa as another FF character for that matter.
And yeah, characters aside, the Prime series could still use more content in Smash than anything, especially in soundtrack. Sylux could bring some of that but on the other hand, Raven Beak feels like he'd be cooler to have than anyone else currently.