I think Byleth and the trio shown off in the first trailer will be the most important characters in the game, they are the pool to choose from when it comes to Three Houses (though if FE gets a DLC character then Sakurai should totally just go for pure fanservice and add Hector since he has a lot of renewed relevance and popularity from Heroes but that's just me.)
Byleth is 99% confirmed an avatar character, since they don't speak while other characters do, Edelgard is referring to him as "teacher" in an ambiguous way rather than referring to him by name, and official marketing refers to Byleth as "your protagonist" rather than "the protagonist". It's not all doom and gloom though. Just look at the pics you posted: he doesn't have (explicitly) blue or white hair, he has a unique color palette, and he has a neat little dagger that can possibly be a part of his moveset. Also, Three Houses seems to have
Combat Arts like from SOV, so he'll have a plethora of animations to choose from other than standard attack animations. We'll have to wait for the game to come out to determine if he's the next Robin or Corrin character-wise though.
Byleth's real draw, however, is being the teacher of the 3 main characters (
Edelgard,
Dimitri, and
Claude). These three are likely a package deal, so why not implement them into 1 moveset a la Pokemon Trainer? We get
axes (Edelgard),
lances (Dimitri), and
bows (Claude), a weapon-variety enthusiast's wet dream. Also, we'd get racial diversity, sexual diversity, and most importantly, hair color diversity.
I'm gonna wait until the game actually comes out to support these characters (even then I'll probably just stick with Hector), but just from analyzing the one trailer, I wouldn't count them out just yet. Could be a real improvement compared to the previous "obligatory MCs" Sakurai has chosen from when it comes to relevant characters.