My suggestion should it not already be implemented is to have zones with elite monsters and / or give bonus experience for hunting tougher monsters so that classes and characters which tend to use single target skills and abilities gain an equal foothold while leveling up.
What I mean is that currently everyone is looking for AoE skills for faster leveling. I personally wish to fight only a single target at a time [Example: Rogue class is better at hitting a single target rather than a bunsh of targets]. But I cannot build my character that way as I fear loosing out on leveling speed. Hence I wished to suggest a bonus xp grant for fighting higher level mobs.