I think the biggest issue you have to address is time. A good RPG takes years to fully develop. A good MMORPG takes many more years. How many will still be into GPU mining then?
What you're saying sounds a lot like entropia online. People invest money into a what is, basically a casino, and some lose big returns, most make small returns or break even, some win big returns. It's a cool idea, though, and it works with Entropia as it is addictive as balls. Instead of money, they're investing hashes, which they could go out and invest in a pool on their own or risk it in the game.
Unlike Entropia universe you do not need to invest any money, the game would be free to play and fuelled by the coins mined for both players and owners (that gets a % of the coins mined and transactions in game) as if the game is popular and stable more players will come and the value of the game coin will increase. And again it's not a casino, it's a real rpg system, at least it has the barebone elements of it, but not at all a casino.
There would be a player based economy on a layer to acquire specific items to improve their gameplay speed and effectiveness, all on top on the baseline coin mining within the 3d environment.
While it's true that mmorpgs takes years to develop, nowadays we have toold like HeroEngine, so you have all the technology ready, just need to integrate game assets, gameplay and the cryptocoins system (you can start with a limited amount of gameplay/textures and expand with progressive patches). I have a quite reasonably good idea for the tools, the only missing puzzle I'm weak is if it's feasible and how to adapt and integrate the cryptocoin wallet/generator on top of an existing set of technologies.