Somewhat related to your comment about alternative XP rewards.

I like the idea of XP stones being a kind of resource for crafting or other areas, because it's another good way to link mob killing with crafting. Generally mobs are only good for dropping potentially craftable goods, but this way players will actually need to go out and PvE a bit to progress in crafting. As long as it's not too grindy (example: you must kill 100 boars to craft 1 boot), I would find it a welcome addition to an MMO.

There's even more possibilities out there too. I'm thinking about actually having level experience being converted into "plot experience" where what you do directly effects a plot-stream. You can level up a different "arc" of the plot-stream in order to alter the course of the plot. Essentially what you do/kill, where you go are going to "level up" the plot if you will. As if the story itself was a skill-based character. Just a random thought.