One of the more interesting experiments Blizzard is conducting in Legion are pieces of repeatable stateful solo content. Repeatable stateful content is interesting because previous runs impact future runs. The best examples are boardgames like Risk Legacy.
So far, Blizzard has included two such pieces of content: the Withered Training scenario in Suramar, and the Deaths of Chromie scenario.
In the Withered Training scenario, you start off with some Withered elves and explore a dungeon. You come across chests, and have to "spend" some of your Withered to retrieve them. However, these chests unlock new types of Withered in your next run. Chests can only be retrieved once, and then don't appear in future runs. So fully completing the Withered Training requires several plays. Each run you may go to a different area and collect the chests there.
In Deaths of Chromie, the limiting factor is time. But after you complete areas, you unlock shortcuts that allow you to make your next run faster. There's also reputation, making the scenario as you do more and more of it.
I think these are really interesting pieces of content. Repeatable content is always good, but the stateful part makes it more interesting. The hard part, I think, would be coming up with a reason the content is repeatable and stateful. The Withered scenario is an artificial training scenario, and Chromie involves time travel. Time loops like Groundhog Day are classic repeatable content.
Perhaps a large dungeon with many floors might work. You could do things like unlock shortcuts, or perhaps killing a boss gnoll causes all the gnolls to flee and be replaced by spiders.
I wonder if you could do something with repeatable stateful group content. The problem here is reconciling state. If someone who's done the scenario 100 times groups with someone who's done it 5 times, who's history is used? You could do like raids, I suppose, and simply default to the group leader.
In any case, these scenarios have been a neat part of Legion, and I'm looking forward to seeing how Blizzard expands on them in the next expansion.