Duly noted about lack of production buildings on island cities. I'll see which of your suggestions are best, but I don't like making the Feitoria too similar to the dike, so we'll go with merijn_v1's suggestion for that building.
In a way many of them do give stability, since increasing happiness in your cities improves your stability. I think giving not allowing cathedral cities to declare independence wouldn't be too powerful. You can only build them in 1/4 of cities (unless you're spreading multiple religions.
Making little buildings which add something onto the main building are sort of annoying IMHO. I know I did this with the religious buildings, but it still annoys me to actually play with it
and I'm thinking of simplifying/reducing the religious buildings a bit too.
I thought about making units require buildings. I guess this is prevalent in Fall From Heaven, and it plays alright. I don't know if the AI will deal with it very well, and it's a pain to test. Maybe we could just limit the most powerful units...
Is the problem with the buildings themselves or because they are too weak for the cost?