Borrowing my experience from EVE-Online, they had a system of repeatable quests where for each different "class", there is a set of quests for a given range of levels, and one can just keep doing those to level.

I do not see ARR needing anything fancy, maybe 5-10 quests per 5 levels, per class, should be plenty to make it work. This should keep those that rather solo busy (personally I do not see why in a MMO), and the party players will always have their parties to find.

jc