Given what the vision of the game seems to be, I doubt changes to the game (whether there are expansions or not) will change the game experience from what we came here for in the first place.
I am only afraid of power creep rendering content I loved useless, and there are better way around that than Vanilla or Progression servers. This strays away from the main topic, but keeping the old zones meaningful (specific crafting materials, events, quests) and somewhat challenging (the way Destiny is capping damage on low level mobs, adapting damage taken from them, for exemple) would work, I think.