Actually I was thinking about this quite a time now.
The problem is that MMO game is so dynamic - building a blockchain protocol for the game is rather a complex task.
If each action of the player is a transaction - you have vast amount of data for each second. And above this you should have block confirmation time
~1scecond - 20ms (for technical time frame rate)
So it should be PoS or something.
Next thing is blockchain size - you cant keep it from the very beginning - too much data. So genesis will have to be moved to father blocknumber.

If you have ideas to discuss please contact me.