I seemed to have missed the news when it first came out, but I just now learned that Fire Emblem: Three Houses is getting DLC that adds a 4th house. There's a chance that, just like with Corrin, Byleth was added to draw attention back to the game for this Expansion (and for those who never played it the first time). Byleth doesn't come out until the 28th and this Expansion isn't ready until the 13th of February; that's pretty close and I can definitely see this as a marketing strategy.
I know some people will go 'well that's obvious' but I mean, it isn't always to promote something. Sometimes its genuinely just to add a character from a game that was well-received (Like in the case of Joker, Terry, and Banjo). The timing of this though, so close to when an expansion will release, and at the end of a pass that was made up of all 3rd party characters at the time, means it was probably intentional and not just because they wanted to add him.
That being said, we've got 2 different Expansion Regions that are supposed to add a lot for Pokemon Sword & Shield coming. One over the Summer and another in the Fall. I'd be willing to bet that one of six remaining characters will definitely be another Pokemon; this time from Sword & Shield.
The Fire Emblem DLC adds another house and a bunch of new characters, but our character is Byleth. That means that if a Pokemon Sword/Shield character is added, it doesn't have to be a new pokemon or anything; it could very well be another starter.