Moving off-world could be the logical progression of the rules.
The two choices, imho, are E.R Burroughs' Barsoom or H.G Wells' War of the Worlds genres. People's individual preferences are going to differ here; I would prefer E.R.B myself.
Black Hat's 18mm rules "Martian Empires" actually blends both genres together in their fluff and lists; it's the one thing that I found difficult to identify with when I bought their rules, I'd much prefer one or the other ... but not both together.