So theres a FPSC project by Rolf that used an underground sewer system to access parts of a large city.
It was a very clever system and probably the best way of attempting something like an open world game in FPSC.
FPSCR will be much more suited to large maps but that won't be available until end of the year.
If your new to FPSC spend your time learning the tools, scripting language, some basic modelling and texturing skills which are all required to make any successfull project.
There is a stock NPC script called passive or you can use follow to have them following waypoints.