I think I now see what the big difference is here - you may correct me if I'm wrong vedrit - but there's talk of two very different situations going on in this thread, one being the idea of making a for-profit commercial game and the other being more of a personal, "hobby" project. When I first heard that your idea won't likely be GPL I had assumed you were shooting for the first scenario, which I apologize for.
A full-blown commercial, for-profit game obviously has to follow certain rules because it has to be successful, so that the developers (part of a large, full-time team) earn a living.
A smaller hobby project, or something done mostly for fun, doesn't absolutely have to count on being a huge success and take on all the competition, because the smaller group of developers who make it don't depend on it for their income and to pay their living expenses. The advantage, though, is that such a hobby project - because it doesn't demand world-class success - doesn't really have to follow any rules that the first scenario/case would, because it isn't critical that it attract and appeal to the majority of gamers out there.
Regardless, I do have other models I've made and other content that I haven't suggested for UFO: AI, some of which I guess I could offer to your project, although I admit it wouldn't be quite enough for a complete game.
If you consider switching engines, I've been working at learning to code in C++ with the Irrlicht engine for some time now, it is possible that down the road (but not right away) that I could contribute a little coding work - I could certainly use the experience - and that engine has been used in network and multi-player games before, including at least one MMORPG that I've heard of, as well as being available under a very liberal license fully compatible with both GPL and commercial works.
Right now I have other things on my plate I need to digest first though, and if I do help out in such a way in the future I would need help, because if I was the only coder/artist/contributor it wouldn't get done for a long time.
If you can find other people willing to help in various ways, including more content-makers, you might just be able to get your idea off the ground and going.