Good idea!

Hmm....while it would feasible add the entire 'traditional' eight aedra pantheon with their various incarnation that appear in a variety of different cultures, I would wonder upon things like gods who are unique to specific cultures like Trinimac, Y'ffre and Rahjiin who are not considered to have a counterpart or similar entity within the eight great aedra.

How would they become available? Or should they be available at all?

At least daedra are practically universal and their identities hardly disputed most of the time (if you exclude the Night Mother's identity and aspects of sithis and maybe a few aspects in Redguard/Skaal culture. But then again, I'm hardly an expert.). On a similar note, while daedra are typically viewed as evil or undesirable by the great majority of cultures, the argonians and the ashlander dark elves in particular have a certain acceptance toward daedric influence, and may not respond negatively to any daedra worship.

There are many technicalities and details that have to considered before any worship of X deity in Y zone are implemented.....unless they want to ignore various aspects of lore.

I had thought the same thing a while ago. It would be nice to have some kind of devotion toward a specific aedra or daedra. If no skills or stats buff, perhaps something cosmetic or unique from each entity. Regardless of what they would come with, I think it would just be a really nice addition to the world.

Just another vote toward the devs for this idea. I also wouldn't be surprised if the devs already have thought about this and have it penciled in on a drawing board somewhere.