Yes, I agree, a coin connected to a game that already has a massive following could be huge, but I think it would be difficult to implement.  Huntercoin works because, as far as I understand, the game states, players - everything is encoded in the blockchain.  I wouldn't necessarily call Huntercoin fun (though it can be).  Movement and actions are transactions, which update frame by frame with the mining of new blocks every minute.  It is something best played passively, in the background while you work or surf the internet.  The game aspect is more of a vehicle for realizing the concept of human mining, which works brilliantly.