I thought a bitcoin-themed RPG would be good, reminiscent of the final fantasy series. The thing I think that would be most interesting is if the BTC in the game represented real life BTC. Essentially, this would be a gambling RPG...
1) To begin playing, you must deposit a certain amount of BTC. BTC will be the in-game currency, except the BTC used/spent/gained in the game will be real-life BTC.
2) The incentive to deposit BTC at the start influences your ability to complete the game. Weapons, armor, items, etc. in the game will require (real) BTC to purchase. Thus, the more BTC you deposit, the easier the game will be, though the difficulty should plateau at some point -- somehow, there should always be an element of luck involved where the player can lose what he has invested.
3) The game will have the potential to reward players with real earnings. For example, whatever amount of BTC the player completes the game with will be rewarded to him after the game. If the player loses the game, he will also lose some amount of BTC (either all of it as in "game over," or he may be able to restart from some point with only some of his BTC missing).
4) The game will have to be well balanced mathematically to ensure a proper difficulty level. After all, we wouldn't want someone to easily manipulate the game so that they can acquire a massive amount of earnings without also facing a large risk of loss-- the point is that the game should be fun to play, but it should also allow for a real chance of earning real life BTC.
It would be one hell of a project to design, but I bet a lot of people would play it. I know I would.
Edit: Another idea would be to adjust the difficulty of the game based upon the amount of BTC initially deposit. For example, let's say I deposit 1 BTC. This may give me the potential to earn 2 BTC if I complete the game. But let's instead pretend I initially deposited 2 BTC, and this would give me a potential to earn 4 BTC if I complete the game. Because my initial deposit is twice as much and I will be able to buy twice as many items in the game (at least initially), perhaps certain factors of the game could increase in difficulty to match the deposit. The goal of this would be to keep the game equally difficult no matter how many BTC are initially invested. So, while I may be able to buy stronger weapons and armor to begin, the enemies may be more difficult.