To me, high level play is the ability, and even the need, to travel far and fast. And sometimes near simultaneously. And being proactive about it too.
That is a very personal definition, though. It doesn't follow that "they can't do high level content unless the map is bigger." You really mean "they can't do MY IDEA of high-level content unless the map is bigger."
I don't see them doing a stand-alone high-level sequel product, for the same reason WotC doesn't do adventures like that. I hope they do a sequel, but if they do it'll start at level 1.