While I'm sure it's difficult to separate the code combining the skill lines into a class, I think that subclasses would be interesting if done in a more tamrielic storyline way. What I mean by this is that if they ever did give us subclasses, they should have it more like a quest where you work with an npc who is that class, like they have tell you about their class in a way that gives each class more lore behind what they do, how they learned their craft, and then after you finish their questline they tell you that they can teach you one aspect (re: skill line) of their class. This would be a permanent addition that you couldn't change in any way. The players would pick one class firstly, and then one skill line, basically.
The questline could be done by you meeting the illustrious Madam Whim outside of some building where she bestows a gift of a newfound experience and allows you to choose which class you'd like to learn the experience from. Through some applied daedric magic, your character is whisked away to some realm where they and the member of said class find themselves stuck and need to escape. Easy, writes itself almost. This would allow you to do it the quest on separate characters, and only once for each. Now there could be some token to change the skill line, but it'd only work for that one skill line (no changing your actual class), and have a restriction on the token where it can only be purchased once per 180 days or something. Thoughts?
PC - NA
CP 1125
Veric Blackwood - Breton Magsorc DC
Xhiak-Qua'cthurus - Argonian Frost Warden EP
Kujata-qa - Khajiit Magplar AD
Suunleth-dar - Khajiit Stamblade AD
Teldryn Antharys - Dunmer Flame DK EP
Strikes-With-Venom - Argonian Poison DK EP
Rur'san-ra - Khajiit WW Stamsorc AD
Ilianos Solinar - Altmer Stamplar AD
Iscah Silver-Heart - Reachman Magden DC