There are a few WH40K mods in the works. There's one for L4D and there are a few for FO3 (mainly Space Marine power armor and the like). There's also a BFG mod in the works for Sins of a Solar Empire. If this mod is done, I suggest you not make it a standalone game. THQ has the exclusive right to make games in the W40K universe, free or not, and they might take offense. If its a mod, you might be okay. GW's IP restrictions are incredibly restrictive, however if you follow them chances are GW won't be angry and smite a BFG mod, especially since BFG is one of the properties they don't care about that much.