I was looking at the Hellmouth concept art and an idea occured to me. What if gravity affected gameplay? This could be a very interesting mechanic, especially on the Moon and Europa. The Moon has about 17% as much gravitational pull as the Earth, and Europa has only about 13.4% as much gravitational pull as Earth. Who needs jetpacks?
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I think it would be a pretty cool mechanic if every planet (or maybe just some of them) had varying gravity levels to them. I personally haven't seen it done, but then again I haven't really played all that many sci-fi games.