I'm not sure if this has been suggested before, but I think it would be cool to have something like the slayer skill or farming contracts (although not actually a new skill), for other skills. It could be an NPC that gives out tasks for different skills. I think this could take out some of the monotony of the grind for skilling. For example instead of spending countless hours at motherload mine, it would be a nice change of pace to get a mining contract to mine a certain amount of an ore and turn it in for possibly an additional reward. Even if the exp rates were similar to current methods, I think it would make skilling more exciting.