Ah, not the S-support CG, I mean the actual drawing Edelgard does of Byleth during CF. It's m!Byleth regardless of who you're playing as, but the S-support CG does change. It's weird!
I really hope this is it! It's definitely a long shot, but I loved how in Astral Chain, the protagonist you didn't pick was your twin and had a role in the story. I like it a lot when games do things like that, so while I think it's unlikely, it'd be really neat to see.
Edit: as for the sexuality thing, I think the "everyone is bi is pandering" is sort of a disingenous argument. Like it or not, discounting Echoes, the last three mainline games have all allowed marriage between heterosexual pairs for the player character without limit (even if there really, really should have been limits) and that's already pandering to the presumed-straight audience. I think it's a bit unfair to say "everyone is straight for the Avatar" is okay but "everyone is bi for the Avatar" isn't. At the end of the day, it is a video game, and characters are made to appeal. I definitely agree with @Interdimensional Observer's take on it - realism would be nice in other genres (personally I like the Dragon Age style best, and I do want more varied relationships between NPCs too), but FE hasn't had the full lineup available for everyone like, ever.