You can replay pretty much every mission in Rise of Iron. I love both of the Gjallarhorn missions, and I was worried that my only chance to replay them was tied to running them with folks who haven't completed them yet... But they're all available in the director, even after you've completed the quests!

I also love that there is a challenge in the RoI Book that tasks you with doing the story missions on Hard difficulty, which is a neat touch.

So yeah, now if only we could run missions like Ritual of Sacrifice, or the Subclass missions again, I'd be set...