I'm new to 1C and an Enthusiastic Off Road gamer.
One of the Best Off road games is Codemasters I4SANE made way back in the 1990's, it has Fully roam-able maps, but it didn't at the time have a Free Roam Mode, you had to simply start an Un-timed Off Road Race, on I4SANE race tracks had NO Boundaries.
So if on Off Road Drive you can leave the track without Resetting or hitting a boundary, technically you already have Free Roam and as you all say about this map editor, you could create a map how ever you please, with Free Roam as a editor mode or as an un-timed race..
Hopefully this helps to calm your minds
