I don't much care about Lyn, honestly. I think she'd have a lot less to bring to the table compared to other GBA era characters like Hector or Ephraim.
That said, I'm all in for just flooding the roster with Fire Emblem characters. There's a ton of potential with that series, and while I'm stoked to see all the characters we do have and I feel like all of them are worthy, I can see why people would be irritated that some of them are too similar. We can stop adding the Marth-inspired ones now (Though keep them all, just don't add more). Let's add in Lance/Axe/Bow users. Dancers and Taguels and Manaketes (Corrin doesn't count). There's so much potential here, especially if Smash tries to incorporate the ideas from these movesets from FE Warriors. I'd be so in for all of it.
Were I to choose, myself, to add someone new from each thing...
Sword - Sigurd, complete with horse. Go all in with it. Make something completely unique.
Axe - Hector. Slow, but strong.
Lance - Ephraim, with Eirika as an alternate costume. I know she uses a sword, but oh well.
Bow - Niles, with Nina as an alternate costume. Gotta get best girl in Smash somehow
Mage - Sigh... I GUESS Celica... I don't like her as a character at all, but I do like her design a lot and Erica Lindbeck is bae
Beast - I suppose Panne? But only because I can't think of a better option. Demon bunny sounds like it has potential
Manakete - Obvi Tiki, though I wouldn't be against Nowi as a costume or something. So long as it isn't weird British young Tiki from FEWarriors