I think the best option would be to add ways that make leveling more difficult but overall much faster if done correctly. I'm not exactly sure of a way that this could be done well though.
An optional buff that for example might increase your XP from mobs by 5%, reduce your damage by 1% and increasing damage taken by 4% or so per stack. Each mob you kill without any assistance adds to this buff and cap it at 50 stacks or so, resetting on death. Encourages players to be careful while leveling, adds danger and has a notable detractor if a player dies (which at the moment is non-existent).
Another idea might be to implement solo scenarios for leveling. Already had them on the 5.3 isle, having them for leveling could be interesting. Sort of a difficult but one time only story.
Finally as was mentioned above, zone wide mini-bosses would be kind of cool as well. To do it well though you would need to have them be phased after you get too many levels above their appropriate target, just so bringing a max level player doesn't become the intended way to kill them.