Here's my thoughts on it.
If the next Smash in 5-7 years really does cut back on total characters, I would honestly recommend keeping Marth, Robin and Corrin, as they are all unique movesets that also happen to represent the best selling games in the series (Marth was the star of four separate releases, and Awakening and Fates are the only titles to break 1million copies) and Ike sheerly for his obvious popularity as demonstrated in FEH. Then, for the sake of variety, I would use Roy's moveset for another swordsman, and then one unique moveset for an entirely new addition. That would bring the total to six, which isn't overbearing but still a decent representation of a series with 16 canonical games.
I know Roy fans would hate it in particular, not to mention Lucina and Chrom fans (me), but replacing the old guard with a new guard after the explosion of Fire Emblem's popularity makes the most sense to me. Corrin may be a dumb character from a dumb game, but it's still a very strong-selling game with a major fanbase despite its well-documented flaws, and she/he has probably the best rounded kit out of any FE character.
Personally, I'd go Marth, (OG gen) Robin, Corrin, (3DS Gen) Ike (GC/Wii Gen), Lyn (Roy's kit, GBA Gen) and one of Edelgard/Celica/Anna as the new member. That's an even split of male to female, no generation of FE is overrepresented, all the characters are well known in and out of the FE community, modern "anime style" FE fans are happy, classic FE fans are happy, and everything's fine.
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That said, I would still try to wedge Lucina in as a Marth clone because I don't like that tipper anymore and I really want Lucina's moveset again. Plus she's Lucina.