My most wanted is Alear.
Hear me out: they can "cut" all Fire Emblem characters and just leave Alear.
All FE characters that appear in Smash also appear in Engage as Emblems. Therefore, they could do the following, have a default Alear moveset focusing mostly on punches and lasers, with sword only for specials. You would select one of the FE characters as an Emblem for them in the CSS, and they would have a bar that when filled up enables makes them engage with the selected Fire Emblem character, giving Alear their moveset but as a powered up version for a limited time.
I think this is the best compromise. Nobody is really cut, but at the same time there's only really one FE character in the roster now.