As the title says: on the main map right should be possible and when one right-clicks over the adventure location sign a small preview should appear. I'm now at a higher level and I want to replay some of the adventures and is quite tedious to have to enter the adventure to then exit it.
Preview meaning it shows the map (first only?), shows the story, or what precisely? And where? Great idea for sure. I know I would make use of it, regardless of exactly what it shows you. But deciding exactly what to show and where/how to display it is pretty important. I cannot think of a nice way to display all of the information I personally would need, other than allowing players to make notes on each adventure for themselves (ie "- bring lots of WWE for this one," or "Resistant Hide on 3 Mages, gg" and maybe "Oh god, the armor! Burn it with acid!")
Just being able to see the first campaign module page and description of the first battle would be pretty cool, to me. That might be enough to remind you what the adventure was before you head into it again, maybe?