Which GHB is next?

a 'schedule' does not exist for this information and there are even a few GHB units that haven't been revisited even once (Xander, Ursula, and Cammus specifically) and we have a second return of Narcian so there isn't even a way to predict it sorry.

IT'S THE BLACK KNIGHT!!!! I wish.

Just kidding. No one knows. They haven't announced anything yet.

As others have said, we have no actual info on this. I would guess we get a new GHB in September and 1/2 repeats.

On the repeat front, it would be cool if they bring back F!Robin since she is the only source of blue tomebreaker in the game and I don't believe we've repeated her yet.

If the next TT's theme is Sacred Stones, maaaybe it will be Lyon or Caellach. Lyon's personal tome was mentioned at the end of paralogue 10, while Caellach was a boss in the same map as Valter. This is just speculation based on the recent SS banner and paralogue 10's dialogue. They might as well throw Black Knight's GHB or freaking John Cena. Ehh, who knows.

No existing schedule yet, maybe in the next version update (so it can be datamined).