MAngband

Just make vanilla have multiplayer capabilities.  Of course, certain new things are needed like chat and trade and balance maintenance.  Then, if people wanted to add all sorts of different "strange" things, they could make their own variants.  I imagine that sticking to vanilla would cut down on development time as well.

mangband actually doesn't currently stray far from vanilla.  However, the changes required under the hood are a bit more complex than you picture.  Part of the process for 0.8.0 is updating mangband to angband 3.0.1 (including monsters, artifacts, and vaults).