Elder Scrolls was always a game with no actual classes defined, as your play style (and skills you choose) defines your class for you. I was a bit disappointed when I started playing ESO that I was forced into one of the four classes, and I was always dreaming of some form of true hybrid builds across classes, which I can experience playing the same character.
What I would actually want to propose is to make it other way around - it is not that you choose a class that defines your skill lines, but skill lines define your class. You have to pick several "class" skill lines (probably, also "weapon" ones) to choose your "class". I can choose any skill lines from the list of available, and I can change them any time ("re-morph" them).
Let us say if I choose Shadow, Assassination and Siphoning - I am a Nightblade, if I choose Shadow, Assassination and Daedric Summoning - I am a Daedric Ranger (or whatever). In this case classes are simply "labels" over your selection. They may have certain bonuses (e.g., Nightblade may have a very useful bonus for extra stealth damage) through which ZoS can buff/nerf certain combinations of skill lines to make it (at least somewhat) balanced.
Probably, to make it more manageable, selection should be limited to 2 skill lines (+ 2 weapon skill lines).
Then, ZoS can introduce a new skill line every major update, and I will be looking forward to it since my main character may benefit from it (or, at least, play with it for a few weeks), unlike a new class that my only character (including me) cannot enjoy and does not have an incentive to pay for.
PS Apologies if it has already been discussed and proposed before, but the more I am thinking of it the more I like it. Could not resist sharing these thoughts ;-).