I don't have a direct experience with MUDs (multi-user-dungeons) but they seem hella cool. Essentially they were multi-player text based games.
That interests me because I think theres potentially a type of interactivity you can achieve with text-based games that you can't achieve with standard graphical ones. This is because nearly everyone has a familiarity with writing and reading text, whereas very few people have the ability to "write" the complex visual data (and code) otherwise neccessary for games. Things like minecraft of course do come close to all players being able to write, but there's still a limiation.
Of course the problem is if you remove the limitation it becomes incredibly hard to parse what the player actually intends. It's this wierd paradox where by expanding the visual of a game, we can constrain the inputs and it still feels more interactive.
Anyways, an interesting way to get around the fact that it's hard for computers to parse what players intend is to make it multiplayer, such that it'shumans parsing what the players intend.
This starts to look like RPGs, but put into a data and interaction structure such that it looks, feels, and has all the power of, a game. These features could look like systems to keep track of the map or an inventory, or get as complicated as writing scripts or code to manipulate the world.
Postcard NetGames are a phenomenon I just read about, which were kind of like this but much large scale and lower latency. Players would write in to the publication and the publication would write back every month with what happened.
What I'm imaginign is a little bit like a democratized version of this. Anyone would be able to create a game, set it's time constraints, and begin building this world. At points they could fork their game worlds, or merge in players or data from others.
In the netgames at the end of the year they would publish a book of everything that happened in the shared world. I think this could be fairly easily replicated given the publishing abilities today, and would be very very cool.