One thing they could do, is put deminishing returns on the same action. Say there's something you can do in game, and the first time you do it, you get 100xp, but then the next 100 times you do it, you only get 90, then the next hundred, 81, then 72, 63, 54, etc until you've performed this action enough times that you only get 1 or 2xp. Deminishing returns coupled with increasing requirements for leveling would naturally push players to change their play styles over time. This could also allow for different types of tasks having different xp characteristics. You could have some that start high, but drop quicker than normal when repeated, and some that start low, but have very slow degradation over time.