I don't really get the idea of implementing the blockchain to the mmorpg...only for various ways of donations, maybe...but that's not, what you want to hear, isn't it? If you do so - think not only of the game itself, but also of a generous and multifunctional coin, if it's going to be used outside.
Just to get this straight:
There is an in-game currency which is a completely normal cryptocurrency, which can be traded on exchanges and so on. Players receive them as a reward for quests, devs make the money for server costs (etc...) from ads. Furthermore, at some stage in-game the players would be enabled to mine the currency (with their PC, but there is also an in-game interaction, with a dwarf or whatsoever).