I’ve been considering doing a quick MUD/web game (I’m thinking no more than a few months to get at least something online) and to save time I’ve been looking around at some existing systems/settings that I could adapt.
The Pathfinder Roleplaying Game is a fork of the D&D 3.5 edition rules that has grown into a large product line in its own right. It’s a good candidate for a MUD as the D&D style system is nice and familiar and there’s a a huge amount of Pathfinder material that I could use.
The mechanics are all freely usable under the OGL and they also have a community use policy which allows you to “descriptively reference trademarks, proper names (characters, deities, artifacts, places, etc.), locations and characters” from most of their products, provided it’s for non-commercial use. As far as I can see this would mean I could set the game within their world as long as I wasn’t copy and pasting stuff from the source books. Actually, as far as the OGL content goes you could copy the text verbatim and they have a system reference document available online which would be ideal for that.
The community use policy also allows you to “descriptively reference dialogue, plots, storylines, language, and incidents” for use in “campaign journals and play-by-post or play-by-email games”. It’s probably a stretch to include a MUD in with play-by-email games, but if you did it would allow for converting their published adventures into content for the game which would be neat. I’m not sure how you copyright a plot or storyline anyway (or game mechanics for that matter, hello OGL!) but it’s nice for them to give explicit permission I guess (cough, Wizards of the Coast).
The second system I’ve been considering is Dark Dungeons which is a retro-clone of the Rules Cyclopedia version of D&D. The author has placed the non-OGL parts of the game into the public domain so there shouldn’t be any issue with adapting the material for a MUD.
There’s also a version called Darker Dungeons which makes some “house rules” style changes to the original such as swapping the elf and magic-user and allowing druids to start from level one. I think the changes that the author has made to the original improve upon it without altering the basic feel of the game and I’d be tempted to use this version over the original.
The BECMI version of D&D was my introduction to RPGs so Dark Dungeons has a strong retro appeal for me. The mechanics are simpler than Pathfinder so it would be easier to get something up and running but there’s no setting or theme material with Darker Dungeons so that would need to be created.
A compromise might be to use the Dark(er) Dungeon rules but set the game in part of the Pathfinder world, or stick with Pathfinder and start with a subset of the available races, classes, spells etc.