I'm not a huge fan of FE, so maybe this is just my own biased opinion speaking, but I think it's possible that we might not get any FE characters.
The last new FE game to get released was Fates. Normally, I'm sure we'd all be claiming that a character from Fates would be likely to get in Smash Switch, but since Corrin got in as DLC, the odds of someone from Fates getting chosen is rather slim.
SoV was released last year, but that was a remake of an older game (and if I remember correctly, the original was poorly received in Japan). Remakes never have as big of an impact as newer games. This game didn't stick around as long as Awakening and Fates did, and honestly, if it wasn't the latest FE game, I don't think many people would expect a newcomer from this game.
FE16 being the inspiration for a newcomer is already a stretch, but even if Sakurai had enough material to work with to make a character, there's no guarantee that he'd actually use it. Not every new game is going to warrant a new character in Smash, and FE is no exception. Gen III and Gen V got skipped over, so perhaps FE16 will get skipped as well due to its timing.
Lastly, there's the possibility of a FE character from an older game getting chosen. I...really don't see this happening. There aren't many times when Sakurai has gone way back to add older characters to a popular franchise, and even then, those guys are usually extremely iconic characters who make regular appearances in their series (
). I don't think there's really any FE character who's so iconic that Sakurai MUST go back and reconsider adding them.
As of right now, there aren't any FE characters that stick out enough for me to think they'd have a good shot of being playable. If FE16 doesn't get represented on the roster, I think we can expect a possible DLC character.