I don't see how you can make a list that would include Roy or Ike and exclude Lucina or Corrin. Not only it would be pretty terrible in terms of gender balance, but Ike and Roy are only popular and relevant because of their history with Smash, and if you assume that exists, then everyone else who was in a Smash game should have to be in because of Everyone is Here.
Honestly I don't get the Corrin hate. She's a popular character, Fates is the highest selling Fire Emblem game (counting both versions) and she brings some of the much demanded "variety" when it comes to fighting styles. I can guarantee Corrin is a ton more unique than a character like Hector or Ephraim would be, if nothing else, because Corrin is kind of a Manakete and that brings both a unique look and style. The only thing I can't understand is why the male version is the default one. By any metric she deserves to be in Smash a lot more than Roy or Ike if we exclude previous Smash games, and if we don't then everyone gets to return.
Asking for every weapon type to be represented is like asking for every Pokemon type to be represented, it's not going to happen.
Regarding Ultimate, however, the one character whose inclusion I still think of baffling is Chrom. Awakening already has two other characters, so the excuse of there being many Fire Emblem characters because of the many games doesn't apply. He's visually just a less interesting version of Marth and Ike, without the shonen spunk of Roy or the being-a-woman of Lucina. Gameplay wise, he's the fourth Marth. Popularity wise, he's still not as popular as Lyn or Hector. If he was an echo of Ike it'd be mitigated somewhat, but since he isn't, the only way I can make sense of him being in the game is if Black Knight took the spot of Ike's echo. BK as an echo of Ike, Celica as an echo of Robin, hell, even some weird form of Tiki as an echo of Corrin would all be better. He's the sole character I'd exclude so far.
Okay, rant over. I know that's not really what the question was about, but I had to get some of those things off my chest. Trying to create a Smash roster of Fire Emblem characters ignoring Smash history so far would be hard because there'd be so many variables. Marth would be in, for sure, and I'd certainly include Tiki as she is a popular recurring dragon that would have a completely unique moveset. But after that? Lyn is more popular than Hector, but in a world without Ike he certainly offers more to the game. Veronica is the face of Heroes (the biggest Fire Emblem game), a villain, and a wizard, but that'd probably get somewhat in the way of Robin, who is still the best choice for an Awakening character. Without him, a unique Chrom as the heavy-sword guy could work, but then I'd pass Hector in favor of Lyn. Lucina would get in no matter what, because she's extremely popular and an easy echo of either Marth or Chrom (probably sharing a couple moves of each). So basically, I'd have, trying to be unbiased:
Marth - Tiki - Veronica - Hector - Robin - Lucina (Marth echo)
or
Marth - Tiki - Veronica - Lyn - Chrom - Lucina (Chrom echo)
All things considered... I have no problem with our current Fire Emblem roster if the Black Knight gets added. I still believe Veronica should get in as dlc in the future because Fire Emblem Heroes deserves a playable character far more than Three Houses likely will, but I am content with what we have. Just give me the Black Knight because it's the only way I'll understand Chrom being a fourth Marth rather than the second Ike.