I would love a version of Pirate Fluxx, just because I loves me some pirates, yes I does. Plus, with the current pirate fad going on, it seems like it would sell pretty well even amoung mundanes.
"Spy Fluxx adds several new cards for things like looking at
another person's hand or stealing Keepers, "double agent" Keepers which
work for multiple players (ex: "If this Keeper is in play, both you and
the player to your right are considered to have it for purposes of
fulfilling a Goal")"
Heck, I don't remember what version they were dropped from, but there used to be a pair of New Rules --Government Cover Up and Hidden Agenda. With the first two, you could hide one or all of your keepers from view, with Security Breach being an action card that let you discard either of those from play.
I'd also totally be into a version of Chemistry Fluxx, where the keepers are elements and the goals are compounds. That would be so fun!