Last year we caught wind of a project called OpenMW -- an effort to build a new, open source client for Bethesda's open world RPG Morrowind, with the potential for co-operative play. Unfortunately, while the project is still going, multiplayer is still on the "would be nice, but really hard" list.
Here's the relevant bit from the site:
Will OpenMW support multiplayer? The OpenMW team currently has no plans for developing this feature. There are probably some technical factors about OpenMW that would make it extremely complicated, messy, and painful to implement. However, sometime in the future, some sort of co-op feature for OpenMW could be a possibility. OpenMW will probably never support MMO style gameplay.
The main benefit, going from the changelog, is "a massive performance speedup", but it does mean the team has to reimplement several visual features, including shadows.
Still, it's good to see the project hasn't been abandoned. If you want to check it out, both binaries and the source code are freely available -- though you'll need to own a copy of the game to play it.