[@Mattchstick] I think you're sort of on the right track but what you're getting at is very niche. For a more "standard" tolkien-esque fantasy setting you could do it like this:

You have towns that act as hubs. When in said towns, characters continue to live as normal without their players. Players can come back and pick the character back up whenever they want. When off on a quest with a group however they must diverge from the group, concluding their commitment to said group before they can drop out. This is obviously a very rough notion but I think it makes the point clear. Note that due to the scale and complexity of interstellar sci-fi, that genre gets much much harder in this realm because of difficulties managing playable space.