Personally, I think a fourth level of magic would be nice. Of course, we would need to come up with good fourth level spells before we add the units with these abilities.
I would rather the "tower mages" be actual units, not just a promotion. Also, either summoners or Archmages should be able to become these units, so you have to choose between sorcery or summoning IV spells.
I also think the Amurites should get a new UU: The Cassawallawn. This would be an upgrade of an Archmage with access to all fourth level Sorcery spells. It would be a national unit with a limit of 1. It would need to be the highest level arcane unit in the game. (Perhaps it would be easier to create it with a spell than a standard upgrade. This would also allow for Hemah or Govannon the Cassawallawn)
Also, I would like to see the Patriarch added back as a unit with access to level 4 divine spells and a relations boost with civs of that state religion, but not with the holy war ability. (Although a spell allowing you to "convene a holy council," tied with an "AP" (which would need to be a religion specific like in BtS, seperate from the Overcouncil and UNdercouncil) would also be nice)