This is a list of the things I always consider when creating a religion. It's not about creating original religions but it might help with the basics, the foundations of your religion.
Most designer just have a 'vague idea' of what they want and that usually results in disaster.
The first thing you have to consider is what the role of your religion is going to be. Remember that if there are temples and religions, the player will want them to do something. Religions can also be used as a great excuse for wars and other problems. Then there's the eternal clich of being the 'Chosen One', but I suggest against using that. If you have a monotheistic christianity-like religion there's a great danger that your game will get all preachy and people won't like it because of that.