Or the other way around... with a double-death option. You float around all the time by default, but if you get "killed" gravity kicks in and you fall... however, you would still live, but now with regular gravity on earth and could shoot the floaters and they could shoot you. If you die while on earth then you die for good.
Uncharted 3 Co-op Certification Center: Uncharted.VarHyid.com
That would be amazing doing Snipers again, just a shame ND probably won't do it.