I dont see why you have to stop getting stronger at lvl 99. This isn't exactly a new idea, but why not have a system where, after lvl 99, you could begin to "sell" experience to get small amounts of stats. For ex. if you were the witch doctor you could trade a set amount of exp for a little mana. Then, to make this worthwhile, you could have a few high level skills with high cooldowns that would use mana to do damage. The more mana you have, the more damage. It would provide an incentive to keep playing.
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