I’ve made a mind map of sorts to explore my idea:
What I’m doing is a game where the player is a mining ship that explores gas giants and retrieves resources… The deeper you go, the more pressure there is, which inflicts continuous damage. You can use the resources to fix the damage. After each level, you can upgrade your ship, RPG-style. There will also be enemies (natural gas-giant inhabitants, and other miners). Other gameplay elements can be gas currents (make it harder to handle your ship), and gravity (ship acts like a lander of sorts, low gravity in the upper atmosphere, higher gravity in the lower one). To repair, the ship will have to land (softly) on rocks that float in the gas giant’s atmosphere… They float because, er…. magic! 
Hopefully I’ll be able to think of some more ideas while I’m developing this, but this is a good starting point…