If you're just looking for game mechanic ideas, I had one I called "endless forest."
Ultima Online puts the entire "land" in the client rather than sending it from the server. The server just sends additions and subtractions. That's because Internet speeds in the late '90s when it was released couldn't handle sending the game world over the network in real-time. That limitation is no longer there.
So, what if you too one of the facets and replaced it with a blank land in the client, just terrain with no objects tweaked to line up on all four sides? When someone enters the facet, you procedurally spawn the mobs, trees and other objects in semi-random locations. As folks leave over the edge, they enter a new instance of the facet with mobs, trees and other objects spawned in different locations. There's no limit in any direction. The facet is infinite in size. When no player is in a particular instance of the facet, it saves to disk and unloads from memory (so that world saves don't take forever). When a player enters an instance, it's accessed with mmap() (instant load).
Because the trees are spawned rather than fixed in the client, they can be chopped down. This would allow house placement almost anywhere which would allow for player-built towns that don't have to work around trees. And of course it's a wide open area in which you can create quests and other things in additional to the procedurally generated content.
Think of it as the Ultima Online equivalent of games like Rogue. https://en.wikipedia.org/wiki/Rogue_(video_game)