I can't believe I have taken this long to figure this out:

Morrowind is a perfect example of different skill trees we have forgotten.

Houses (Redoran etc.)

Temple Skill Tree

ALSO

For RP reasons I also think it might be cool to have something like a Grand Inquisitor Skill Tree (like from Cyrodil, Imperials) (picture half rogue, half fighter abilities)

Dwemer Skill Tree (could be several - one could involve using Dwemer Constructs - another could involve using Dwemer Crafting or Spells)

Ayleid Skill Tree (Spells or some ancient lore, or a tree for fighter/mages)

What do you guys think? Feel free to add more!

The possibilities are practically endless, evident by the sheer amount of factions that the Elderscrolls universe hold. Here's a small list of the factions I can think of right now that could be included

Great houses (mutually isolated from each other)

Temple (Morrowind)

The Companions

The Knightly Order (Daggerfall)

Temple (Divines)

Daedric Cults

Witches (though I don't know how relevant witches are in ES now since Daggerfall, not like they were a huge part of that game anyway)

Though some of these would prove to be hard to implement due to their isolation in certain provinces, like the houses and the temple is only part of Ebonheart and thus not joinable by other the other factions until lvl 50. The same with The knightly Order which is only part of Daggerfall.

And I'd also like to see different vampire clans with their own weaknesses, strength and philosophy.

Id rather they added hunting and fishing skill(s) that would enhance the resource gathering of animals and provide greater gameplay experience for the outdoorsmen. To often this area of games is reduced to trival loot drops with no further developmen, when it can be so much more.

most of those are in the ebonheart pact, yeah, not very fair that one side gets a bunch of of them, one gets all of a couple, and one gets even less.