Akin to how Dragon Age video game series were written. For those who are not aware:

The first game, Origins, is a an epic quest told in a relatively small scale (world ending threat playing out on 1 continent within a vast fantasy world). Along the way, our cast meets plenty of colorful characters and we are introduced to the world in a VERY organic way. Once the central quest is done (in a very satisfying way, mind you), the characters just move on with their lives. They might or might not show back up when plot necessitates their appearance. Following games are self-contained stories that are set in the same ''continuity'' and leap off of what was already established in the predecessors. But every game effectively could work as an entry point into the series.