The Alexandrian has published an interesting article on the Hex crawl. It appears much like an endless dungeon, but in the open air. The 13 posts detail his version of how to put together this kind of open world adventure and finishes with a useful set of cheat-sheets. Also referenced is Ars Ludi’s Grand Experiments: West Marches, which prompting the idea for the Alexandrian.
On the one hand it looks like lazy GMing or GM-lite in the creation of a playable world, which is not a bad thing with the complex and busy lives most people live. However, I suspect that there would be interesting challenges in the preparation you could do. It would help develop the GM skills around improvisation and adaption, along with the creation of little diorama-like scenes for the players to experience. On the player-side, it looks quite easy as their is no preparation and all play, but the risks and challenges are such to make the session very intense.
On the other hand as a GM to does limit your ability to develop long term plans for NPC Villains and such. It would also make harder to develop the boarder story elements in the game, as everything becomes player driven, and depending on the whims your players could go anyway from hack’n’slash survival, monster of the week, to delving into alien cultures.
The same idea could be applied to an endless city filled with blocks of punks or citizens who have survived in the ancient arcology or similar open-worlds. For current examples look at Minecraft, No mans sky, Dwarf Fortress, or even one of the hundreds of the Rouge-like games. All of which fulfill the idea of an endless game world.