Slaughter Points - You could make it so that every time you kill 10-20 of a certain monster, you get Slaughter points based on how hard the monster is which you can exchange at an npc for items or skills (whichever you want, or both).
Classes - You can start out with no class then go on to be a swordsman, or an archer, or a mage, etc. Then they branch off into other classes, etc. And you can advance based on what level you are ex. level 5 and you can be a swordsman, level 25 and you can be a knight, level 60 you can be a gladiator, and finally at level 125 you can advance to an ultimate class called crusader. You get the point.
Passive Play - Maybe some people don't want to go on another quest to kill guys and stuff and want to instead to something passive like a trade skill. Well, with this system, you could have it so that you could hire people and pay them to go level up and pay for all their expenses and be like their coach while you do fishing to get the money to pay them so that they can go solve the problem of the evil big guy.
Those are just some ideas i came up with although they are very advanced ones so i wouldn't be surprised if you don't want to use them.