I'm not sure how I feel about this. I'm sure they'd make some interesting stuff up, but it's kinda lame north of the Wall right now. Walkers are dead, wildlings are the Night's Watch. Maybe there's some Children of the Forrest, and I'm sure they'd discover some newer stuff up there. I'd rather a series about Robert's Rebellion than this, but it's not the worst idea I've seen.
And also if people don't want to see this, just don't watch it. I'm sure it'll get lots of viewers regardless.
Rhaegal is actually the undead one that was killed in Season 7. Drogon is the only remaining one alive. And I'd say he forgave, or at least understood Jon, when he spared him and burned the Iron Throne.