How about when the number goes up and has a real-time effect on the world you play in, showing you the fruit of your labour growing as you progress further and further?
I mean that sounds nice, but honestly I don't "progress" nearly as much as I "faff about" so stuff like XP lets me have the illusion of progress while I spend 30 hours roaming around the starting area looking for collectibles. I'm not sure what a real-time effect on that would look like.