Can't you have multiple users in a dungeon in non text based games? See WoW and RuneScape. Roguelikes have evolved from being text based long ago, so I don't see why MUDs have to be.
MUDs (and closely related single-user text adventure games) are a well-defined genre; you can improve various aspects of the presentation, add rich formatting, images, background music even, but the moment you stop using text as a main medium for gameplay, you're simply not doing a MUD anymore.