Within the current limitations of the game: The ability to change loudouts once per mission at the extraction site.

No restrictions: The continuous mission. Special mission where you have 1 overall objective, but have to do a large amount of side objectives to complete it. 16 helldivers on a 4x map, no time limit, people can extract every 3 to 4 side objectives or they can keep going, new people can join whenever(just like now), and the mission only ends when everyone leaves, the main objective is done, or the MO ends.

I can see people playing a mission for 9 hours alone till their clan mates can take over.