It would all depend on the kind of game that is going to be made. If it's a casual non-profit game that you want to addon to a website, I'd do something like a trivia game, or something along those lines.
If you're going to be making a full length game, closer to the normal indie or AAA game, then I would just add Christian elements, rather than have the game be completely based around Christianity. For instance, have the main character be a Christian, or other small things like that. That way, you do not alienate non-believers, and probably make the game better all in all. Although, I suppose that would also depend on what kind of game you had in mind, both story and gameplay wise. For instance, I am developing an action game with my sister, set in a universe that we created. The player character is religious, and is following that universe's equivalent of Christianity. That way, anyone can enjoy the game, and it won't seem like I'm trying to "Force" my religion on others, which is another common accusation from non-believers when they see "Christian games". In the game, he is not fighting for his religion, and the game is not about it, it is just a subtle thing.
Another good way to incorporate Christianity into games is with RPGs, where you have choices. For instance, the Sci Fi game "Mass Effect", by BioWare has the dialogue options to make yourself a Christian. That was the first, and so far only RPG that I've played that let my character be a Christian. I really liked that.