You know... I always considered the whole thing that people thought about that Byleth should be a Pokémon Trainer-esque character with the House Leaders to be pretty dumb from both the aspects of making the character accurate to canon and most importantly from the development resources management perspective.
With that said, I just realized that there is a Fire Emblem character who would work in that way; a non-combatant that stays in the background and commands others what to do, and one who comes from a very succesful instalment that is still maintained to this day.
I can imagine it two ways: The first is we go full Pokémon Trainer and do have three characters to swap from, those being the Askr trio, Alfonse, Sharena and Anna.
The other is to have a full summon moveset, think Duck Hunt's Wild Gunmen, but applied to everything; summoning different characters to do stuff.
Do I think this will happen? Of course not, there's so much resources that would be put into Kiran as a fighter in either way, with the former they would have to develop 3 full characters, and for the latter they would have make a whole bunch of high quality models of various FE characters to just perform one animation; the way I can see saving most time with the latter would be reusing the FE characters we have in Smash already and even then that would look messy, so not an option imo.
Even so, it's justa fun thought I had.
I mean, if anything the real problem is...
And I highly doubt they will make bosses playable characters via DLC, Ridley had to wait 2 games and be shrunk down.