Old Iron said:
I was thinking the adult version if only because there wouldn't be a way to go back to their own time. The downside there is that it's likely to be a not very interesting story unless one of them is attempting to run a interference plan against the other.
I don't see why they would try to interfere with each other, unless they have a very big fight about screwing with the timeline or not, ending up fighting each other. But who's going to be in which side?
Not to mention that it sounds like a cheap excuse to have a serious Nanoha VS Fate rematch.
But having one of the kids get dragged back to Yuuno's time would certainly make for a very... complex story. Since one of them wouldn't have existed in that timeline for the entire length of A's to StrikerS/ViviD.
That or it could go like in DBZ where changing the past creates a parallel timeline, so the one that ends up in the future will meet with the canon characters, and maybe after dropping his tag along back to her proper timeline Yuuno could end up in that timeline's future.
As for the relationships, if Yuuno decides to actually help Nanoha against Vita, maybe she would develop a crush on her savior, only to discover that it's the future verion of the Ferret usually on her shoulder.