There are indeed a lot of possible playable chsracters for Fire Emblem. Lot of characters would make for feasable newcomers, especially at this moment.
If we'd take the general consensus, most people rather want less than more. Roster cuts are in most people's cases heavily involved in the Fire Emblem cast. I think that's rather unfair.
Corrin got lucky to get in, Robin was perfectly timed and wouldn't probably make the roster otherwise, Roy was heavily demanded to return and was popular, Lucina was a very easy addition. Marth and Ike are the most important lords, and faces of Fire Emblem in general. I think all characters bare either one Avatar character makes for a great representation. Both could stay, but am thinking that the next main character could potentially be a Avatar character yet again.
And if we get a new Avatar next Fire Emblem, there's the choice to add him to Smash, and maybe the removal of another Avatar character becomes more likely. That's what am thinking at least. Maybe this character will use tomes and axes for example. Doesn't Robin then become sort of redundant? Then again, the whole Awakening thing. Warriors also had the theme going on strongly... I doubt it's gonna be off the radar anytime soon.
However am thinking the cast of Fire Emblem characters we have now is very strong as well. Fire Emblem amiibo had some of the most utility. Means the core cast reappears in some form quite a lot since their Smash 4 appearance. That's why this whole situation is tricky.
NOW HERE COMES ANOTHER NEW CONSPIRACY! What about the amiibo line anyway? Took them a damn long time to get all of them made. With amiibo being compatible still on the Switch, I doubt these Smash amiibo are gonna be useless the next Smash game. And that's why I have a hard time thinking they won't port / recycle / reuse for at least 75% of the content for this next game. No character cuts, minimal newcomers, but strong DLC support. It seems very easy sales wise.