Bump because of new idea.
An FPS where the setting is in a VR chamber.
You could change whatever the hell you want, including weapons, enemies, friendlies, vehicles, gravity, time setting, location, and more.
You can even make your own weapons and give them to yourself or to enemies. Same with vehicles.
But every now and then it malfunctions, and randomly changes every setting.
So you could be having a slightly normal realistic gun fight in New York City, and the next thing you know, it malfunctions, and suddenly you're in 1130 B.C. in Antarctica with laser weapons killing Russians who speak Spanish that have RPGs.
There would be an option to turn that off if you want though.