If they want to force more Mario antagonists, he isn't totally unlikely. Just aside from the Mario overpresentation, he kinda belongs to the secondary tier of Mario main cast, so that it would be abit weird to not fill in the first tier main cast first.
I see the Byleth case is a reason why King Boo raises his chance for the DLCs. However, it could be fair to say its weird for FE series to not do any villain fighter despite of overpresentation while Mario series already had two.
Objectively, King Boo isn't the typical fighter material, and adding that he isn't the leading boss of the franchise, it's hard to tell if Nintendo want to challenge themselves to create another unconventional character (well, they did it to plant piranha, but again? Who knows). Just before King Boo ever has a chance to become a fighter, I think he could had reprised his role as a boss, or become an assist trophy in pass Smash titles.