Here's just a crazy idea, please don't flame me:

::Buying levels wtih ability/skill points::

What if there was a location with a mad scientist-type wizard. He'll increase your level, but you have to permenantly trade Ability points or Skill points to do it.

Maybe half a level per point until level 10, then after that, three points per level. Skill and ability points that you get by levelling are still available, so it's kind of made up for.