Well then just create quest hubs like in WoW. People get to explore, they get into a new exciting environment every 6 levels or so. Then the quest chain will arc as mini storyline of that area, and conclude just when you're reaching the area's max level.

The pace in WoW is about right, not too fast not too slow, alot of people in middle range.

And raise level cap to 75.

This will allow people to solo if they want. And have quests that require groupings, if they want.

Then add instanced dungeons.

Standard WoW clone MMO.