What about Manaw and Alt Clut, the Welsh tribes?
I, personally, find Manaw to be the best tribe.
I have no idea, sorry, maybe someone more familiar with the Isle of Man's history knows? I thought Manaw was more Gaelic/Irish by the time the 769 bookmark begins, would maybe giving it tanistry at the start be appropriate?
I would like to see them change the tribal Irish to tanistry, with riadach's proposals, while the feudal Welsh keep gavelkind but adding bastards to the mix. Not sure about the tribal Manaw and Alt Clut, tanistry to Manaw and gavelkind to Alt Clut?
At the moment Welsh culture has no effects, I would like to see it get "Illegitimate sons eligible to inherit if realm has gavelkind" and I don't think tanistry should require unlocking late feudalism for the Celtic cultures.
Witenagemot for the Anglo-Saxons would also be nice.