I'm Emperor of Alba and besides the Kingdom of Eire I control the Kingdom of Wales(1). One of its Dukes is the Grand-Mayor of Gwynedd, which has built a lot of trade posts and stuff over the years.
The game doesn't allow me to grant him the kingdom title, saying its type is wrong. But as far as I understood, there could have been republics with a rank higher than duke, weren't it?
I could also revoke his title since he was a rebel, but I fear that doing so would destroy his families and trade posts, even if my intention would be to grant it to another mayor, right?
(1) actually Brythoniaid or Bhretain Bheag, can't remember, doesn't matter