My pick for a Fire Emblem roster (though I haven't played Engage, and I have no interest in doing so):
-Marth (he's not going anywhere)
-Lucina (I feel like having four characters that use Marth's moveset as a base is excessive, to say the least, and I like having an option that doesn't rely on a tipper mechanic)
-Ike (I haven't played his games, and I don't use him much in Smash, but he seems popular enough)
-Robin (Awakening saved the series, by my understanding, and he adds some gameplay variety by focusing on the series' magic system)
-Edelgard (would have preferred her over Byleth)
-Tiki (specifically the Awakening version; she seems like one of the most recurring characters in the series, though I was torn between her or Anna)
Admittedly, my only experience with the series consists of Awakening, one route of Fire Emblem: Three Houses, Fire Emblem Warriors, and a bit of Fates (I tried the Birthright route; wasn't a fan) and the original, so I'm likely not well-informed on the series.