As it stands, you can solo your way to 50 or you can party and co-op your way. I think that is just fine. I choose to solo my way using leve quests and GC leves, others choose to party and co-op grinding and strongholds. You learn different skill sets(player wise not in game) and strategies and both have their place. I don't think there is any need what so ever to change how we can level. They have opened the doors for solo players and team players and that is the best we can hope for.
If anything perhaps adding MORE leves with different stories and more creative objectives. Perhaps leves that focus more on story less on "kill X" before time runs out.
Edit
you can also PL your way to 50, which is completely player choice and really helps with people who have already experienced the 1-50 grind and simply want to get a job to 50 with out the hassle. There are plenty of ways to get from 1-50.
