In order to add diversity to each class, replay value and possibly help balance magic and stamina builds I would like to suggest the addition of veteran class's.
Once a player reaches level 50 they would have the choice between three different veteran class's per class. Each veteran class would add one new additional skill tree with five abilities, four passives and one ultimate.
One class would focus on Stamina, one would focus on Magic and the other would be a hybrid type class. This will allow the Devs to target specific game-play and make balance changes on specific play-styles bye class with minimal collateral damage on other play-styles and class's.
For Example: Once a Templar hits level 50 he would have the choice between the Templar Knight, Warrior Priest and Sun Magnus.
The Templar Knight would add five new Stamina abilities, four passives and an ultimate. The Templar Knight would be the choice for Stamplar players. The new abilities would all use stamina and would help balance the Stamplar without affecting other templars.
The Sun Magnus would add five new magic abilities, four passives and an ultimate. This would be the class of choice for Magplar players and would add new magic abilities that would help balance the Magplar without affecting other templars.
The Warrior Priest would add five new abilities "magic/stamina mix", four passives and an ultimate. The hybrid class would add abilities and passives that would make the hybrid class a real possibility in the game. It would allow the developers to add skills and passives that made this play-style possible without allowing the other class's to exploit it.
The player would maintain all their Templar Class abilities but would receive one new unique skill tree based on the veteran class they chose. This could also be used to make changes to base class skills to affect only certain veteran class's if need be.
Veteran class for the Sorcerer could be Battle Mage "stamina", Necromancer "hybrid" and Warlock or Wizard "magic".
Veteran class for the Dragon Knight could be Dragonborn "stamina", Shaman "hybrid" and Dragon Priest "magic".
Veteran class for the Knightblade could be assassin "stamina", witchhunter "hybrid" and witchblade "magic".
I think this would be a wise move on ZOS because it would in a way add 12 new and unique class's to the game with minimal effort. Players would want to purchase more character slots with more class choices. It would also add replay-ability to the game and add a wealth of diversity to each class.
I would expect this to come as part of a free update. However I believe ZOS could make money by offering special veteran class packages that would change the visuals on preexisting abilities to give the look and feel of the new veteran class.
For example: The Necromancer veteran class would change the sorcerer encase ability to look like skeleton hands coming out of the ground to grab the feet and legs of its victim instead of the weird rock it currently is.
The veteran class package could also include a new costume, personality and mount.