what i'm imagining:
you get a couple missions to choose from, with indicators of difficulty/reward value.

as soon as you enter a mission, the timer starts. you have to be back at the entrance (teleporter? escape pod?) before it runs out.

each mission has three objectives:
1. progress the story, in various ways.
2. recover battery power.
3. find items & loot.
each objective is optional. you can enter the mission & immediately exit again if you want.

exiting rerolls the mission selection.
BUT: every time you exit a mission, your battery depletes. running out is a total failure.
maybe the difficulty increases each time too?