The alternative is to make bitcoin one currency, and have a primary currency aside from that. You could create a full, normal game economy but allow anything sold by an NPC to be purchased for a few bits instead of tons of "credits" or whatever currency name you give it... then players could trade back and forth and they could accept credits, bits, or both for an item. That would incentivize people to put money in (for the reason in my last comment... the chance to get rarer items) while also incentivizing people to play the game because, if they put the work in, they could sell something for real bits and cash out.