Well, for "storyline based zones," the next one up is Faultline at 15. However, 10-15 (I'm assuming you're in that range at least) give you steel canyon and skyway, with the contacts involved there.
Going JUST on storyline zones -
Faultline - 15-25 content
Striga Isle - 20-30
Croatoa - 25-35
RWZ 35-50
Midnighters/Cimerora - 35-50 (think I'm remembering that right.)
You will end up poking around in the other zones to get more content, though. The early levels zip past, the later you'll get a mix of other zones, task forces, etc. to level with.
I'm sure this has been asked before, but what is the best path to taked for leveling
for instance, I start the game with antonion nash (get to level 5)-> head to hollows (complete all contacts)...what would be the next best zone to head to.
I'm looking for zones that have over arching storylines, like the hollows.