I still say there aren't ENOUGH Fire Emblem characters, honestly.
Smash has added Marth, but after that then it's usually just whoever is in the latest FE game. This means beloved characters like Sigurd and Seliph from the 4th game, Geneology of the Holy War, don't become playable because the latest game at the time was Thracia 776, which is why Lief was originally going to be Marth's clone in Melee...except even he was scrapped and replaced with Roy since even though his game still wasn't out yet, Sakurai was in the know about the new GBA game Intelligent Systems was making.
Then America got its first Fire Emblem game, starring the trio of Lyn, Eliwood, and Hector. Lyn was the most popular because you start the game with her even though the game centers on Eliwood later on, but by the time Brawl came out, Path of Radiance released so Ike was added instead (Radiant Dawn too but Ike was in that one as well). Ike is great from what I know, so his inclusion doesn't bother me, but I was annoyed that Lyn wasn't included alongside him, though at least she was an Assist Trophy.
Both Smash 4 and Ultimate averted this, as the only games released around their time were either remakes, spin-offs, or the one new mainline game (Awakening and Fates respectively). The Awakening trio all making it in eventually made sense because that game saved the series from the brink of death, and while many would argue Corrin was included too early at least Fates was still the only new game around Ultimate's release that wasn't a remake or spinoff. Still, I find myself sad that plenty of older Fire Emblem characters aren't in simply because they're not the in the latest game, so any hopes I have for Caeda, Alm, Celica, Hardin, Sigurd, Seliph, Lief, Lyn, Eliwood, Hector, Eirika, Ephraim, Black Knight, or Micaiah are dashed. At least we keep Lyn as an Assist Trophy and Black Knight was added as well, alongside everyone's favorite Manakete Tiki (hoping Anna appears in some non Spirit capacity someday).
Chrom gives me hope that in the future we'll get Fire Emblem characters that aren't from the newest game, though I'm still doubtful.