So, I'm not gonna guess the units in the banner per se, since I know I'll be completely off. However, I'm here to list the possible heroes for this LH (IE the new red hero) and what we may get.
Also, never forget. Protagonists and important heroes only, so people who are borderline protagonists (No Caeda, sorry XD)
List goes as follows:
Alm, Celica , Seliph, Roy, Eliwood, Eirika, Micaiah, Chrom, M!Robin, Xander, F&M!Corrin
Now, given these, you may wonder what the hell is possible. Well, we're getting a water hero (and to boot we know that logic is gone with the elements since LH's like Ryoma who use lightning are water heroes) and thus I have my string of logic.
Given how IS has treated it's LH's as of late. We aren't getting Roy They're avoiding FE6 on purpose. For what reason? I know not, but it's evident. A remake coming out after 13 so they're avoiding it until they get the new art? No clue, but it feels that way.
Then we have Seliph. We've had multiple chances and I also feel that it won't be him. He's had a myriad of chances since we've had a Genealogy banner every so often and the color has matched. So I doubt it'll be him, I truly do.
Alm is a potential candidate, though I'm unsure if they want to go and drop Alm or Celica rn as Legendary Heroes. Feels a bit, off.
Eliwood feels likely given that it's been a while since a FE7 Hero, and he is missing. He may very well remain a water hero as well. Hector is fire (when his axe is lightning) and since Eliwood/Hector are rivals, water fits in my eyes.
Eirika is a potentially cool unit, but it won't be her. I feel it.
Micaiah is gonna be December's Colorless hero, that's just a guess mind you XD
Sure as hell not Chrom, nope. We just got Marth and another Falchion is kinda weird.
Corrin is a potential unit, as is Xander who we have yet to attain (though I doubt he'd be water just like Ryoma), but just as Xander, I doubt Corrin will be water (or maybe they will, I dunno)
M!Robin. After getting a few different units from FE:A and a FE:A GHB, I fee that it'll be him. Just a hunch. But I digress, people tend to hate it when they say Awakening or Fates is an option, but it always will be.
As such, my suspicions alude to Eliwood, Corrin, or Robin!M. You don't avoid Roy so long and then go and drop him just like that. What's your guess (try to be logical, don't ask for Ghebb please) and then your wish. Since, know, we all have that unit we want but know probably won't be a LH.