glass
(he, him)
When you say "the PCs can modify" during play, do you mean that, for example, if they kill someone they stay dead? Or something more meta?4) A mix of all the above. Specifically, a homebrew version of a published fantasy world initially modified by the DM but one that the PCs can modify during play.
Anyway, I had to vote 1. I have my own world, but I only get to use it occasionally - we mostly do Paizo APs, which are of course set in Golarion. These days, I am a firm believer in running published adventures in the system and setting for which they are designed.
_
glass.