Vast open expanses?
Worldbuilding- AKA. subcreation, is an essential feature of roleplaying; a creative endeavour in which many players- GM or otherwise, love to indulge. As readers will be well aware, this places roleplaying games squarely in a cultural trend the 20th century great grandaddy of which is J.R.R. Tolkien's
Middle Earth. (Map via:
geektyrant.)

The history of subcreation in our own culture of the fantastic is not my subject here, but I cannot pass by without mentioning Greg Stafford's
Glorantha. Best known as the setting of the RPGs
Runequest and
HeroQuest, Glorantha is unusual among RPG settings in that it wasn't a gaming setting at all when it was first created; unusual, but not unique- M. A. R. Barker's
Tékumel was the same. The only adventure gaming settings I can think of which might rival these extraordinary creations in their subcreationist breadth and depth are GW's Warhammer Old World and 40K Dark Millenium.