People talk about "West Marches" and "Adventurer's Guild" as formats for connected roleplay sessions where you have a changing group of players. And it works for people, I guess. Haven't been in one, because they usually are also DnD and I'm not a fan.
Here's another format to consider – The Global Response. Basically Xcom or Terra Invicta. There is a centralized threat that is coming in and creating issues, crises, direct harm or other things, and the Organization is responding to these. The assumption is that it is more or less a war, but for reasons (interplanar/interplanetary logistics; secrecy; the old seals are just starting to break) it's not an open-massed-armies kind of war (at least initially).
So the PCs go out, do a job, return with intel, spend their downtime figuring out the threat and their response, set new objectives and try to overcome the threat in the long run.
Pros:
- A built in narrative direction, all sessions tie into the same ongoing goal
- Can have direct conflict or preparation and recovery jobs, and maybe playing different systems for the different modes
- It makes sense that people come and go – it's an explicitly harsh job that people might not be able to keep doing
- Has a narrative endpoint
Cons:
- It's a war story and that's easy to be uncomfortable with
- Someone has to keep track of the larger situation, including maybe a strategic map, intel, research etc.
