Certainly not in a WoW or Wildstar environment, but an idea that I've been kicking around for a few days. This is purely more of an RP / sandbox idea.
What do I mean by dynamic classes? I'll start simple, with ideas based on a game that includes real world day/night and season cycles.
Imagine a Moon Mage, who's power was tied to the moon cycle. Based on the stages of the moon, he would fluctuate between +10% and -10% damage. On actual full or new moons, he would gain another +10% or -10% respectively.
What about a druid or nature mage where the powers themselves changed over the seasons? Plant summoning in spring, animal summoning in the summer, etc. Not just visual changes either, in the transition from spring to summer they would change most of their abilities for new ones.
While I personally love the ideas behind these, I'm aware a greater player base would never latch onto it. Forget who would even play the classes (I would), but how they would be reacted too. How many Moon Mages would be kicked from a group because 'it's not their time'? How many nature mages would be told to 'enjoy fall, because you won't find a single group that will take you in winter'?
Even if every class was dynamic in such a way. 'Ugh, I want to play my Moon Mage but it it's a new moon, guess I'll play my alt for two weeks until my main is strong again.'
Such classes will never happen, but if somehow, some way such things ever came to be.... I think it would be a huge step to placing back the RP in RPG, let alone MMORPG.
For now it'll just remain some pipe dream.