The dungeon has the following structure:
- Explore. Finding some random loot (and traps).
- Do an encounter. Get loot from losers, expend consumables.
- Repeat with increasing encounter difficulty, until the final loot is gained (or lost), then leave.
There are also fairly stereotypical things to do before and after doing the dungeon, but they are not really relevant.
What is relevant is that the physical structure of the dungeon is not particularly important. It simply provides a background to explore and find loot in, a natural sequence to the encounters, and a location you have to travel to and from. Anything that fills those needs can work as a dungeon replacement.
You could be tracking a secretive group on a town or spacestation. First explore the city for leads, more roleplaying than exploring the typical dungeon, though. Then the lead you found leads you to location, where you will have your encounter. Then go look for the next location armed with the leads you already have. At every location the opposition will be better prepared and it is fairly easy to make it so that some supplies can't be restocked while doing secretive things on tight schedule. Eventually you will find the location that has whatever you wanted, which possibly was simply to find and cleanse all the bases of the group.
Or you could be looking for priceless artefacts in the wilderness while dealing with competition and natives who fail to appreciate the favor you are doing them by taking looting their holy sites. Get a likely location for a site, search the area, have an encounter, move to next area and repeat. Obviously whatever you are looking for is in the last location and obviously the natives are getting more aggressive with every site looted and the competition will respond to dead rent-a-thugs by hiring mercs with heavy weapons...
Essentially even without the physical structure of a dungeon you can keep the structure of a dungeon adventure.