I feel that the existing ones should get flavour before more are added; the Muslim ones have a bit, and the Shieldmaiden title has a bit, but I believe that that is it at the moment. Bland, essentially free, opinion boosts don't really add a lot to the game.
As for adding historical kingdom-specific titles, I don't know if that's a good idea, as there's a good chance that some kingdoms will get far more than others (which could be a bit unbalanced, as they'll have an easier time boosting opinion than a kingdom that gets nothing new) because we know more about them and there's a chance that some titles don't make sense if history goes off the rails (e.g. a honorary "Prince of Wales" title that can be given to the King of England's hereditary heir (or the hereditary heir of the Emperor of Britannia, I suppose) assumes that England controls Wales and that Wales doesn't have a holder). However, it is hard to say that for sure without knowing which historical titles are to be considered for addition.