There have been a bunch of MMO roguelikes. Some of the major ones are Mangband and ToMENet (the latter is a fork of the former, made to closely resemble ToME3). There have been a few different attempts to solve the MMO turn-based problem. The most interesting IMO is interhack [1] (not to be confused with the interhack tool for nethack) which originally used "surreal time": depending on whether players were near it other it would switch between them taking turns within the same turn-based game, and running the game logic separately for each player so they didn't wait for each other. There's was a long writeup about it. It's a very complicated system. Never had an active playerbase. nhdaniels told me years later that he considered Surreal Time a failed experiment, and switched to something simpler, which still prevents players from having to wait for each other, but I don't know how it works.

[1] http://roguebasin.roguelikedevelopment.org/index.php?title=I...

Unfortunately the download for Interhack is gone, as is its original website.