Just have the game scan your level, and have the zone permanently scale to that level range. That way you could go for example skyrim-morrowind-black marsh / morrowind-black marsh- skyrim etc.. The only thing ZOS would have to do is make the story and events in each Provence as important as the next(which they should already be doing). Don't make skyrim "slay 10 rabbits" and then morrowind "stop the invasion of the super duper evil man that will [censored] the world up". The Provence's individual story lines should dovetail in some sort of way and be of equal importance