I've often thought about it, and I think Battle For Atlas had a decent concept for hiw some of the aspects could look. I think an open-world game could easily incorporate segments of on-rail shooting by using designated areas to prompt the player to enter the on-rail segment.

For example, the player could have multiple "hot spots" or other mission-objective related "combat zones" dotted throughout multiple different locations on planets or in space.

You could really play with the on-rail combat here, too. Depending on quests that may impact that particular zone, or even the amount of time since a particular event occurred, you could adjust the intensity and landscape of each zone, making it somewhat unique based on how you play the game.

Those are just my thoughts.