Title says it, really.
We know that we're getting some 'unfinished' provinces at start, because they want to leave a lot for future content updates, and obviously can't make several fully fledged zones the size of Skyrim in only 5 years.

So how will they handle that?

Will they just block us off from the rest of a province with an invisible wall?

Will every province require strategically placed mountain walls?

Will there be high level monsters to keep us out?

Or... my worst fear... will the world be extremely instanced?

I have been wondering this myself. I know some places it is just cities that are blocked off, likely by high walls and gate guards, but entire territories need something completely different.