Zombie thread! I was about to post something similar and stumbled upon this topic!
I have shelves dedicated to systems that I thought might survive the RP heavy persistent world environment of a MU*. I've always been curious to see what people want to be translated onto online for their games to use as a code library or something.
I have a few systems that have been sitting on my back burner for a while now:
Cortex/Cortex+/Cortex Prime - The original Cortex is fun, but much more rules heavier than the later generations. I wonder though if +/prime are /too/ rules light for the community, which leads me into:
FATE Core/Accelerated - I really /really/ love the concept of Fate. I've heard though, through from a few people who've experienced MU* play with the system that it requires a pretty heavy Staff presence to keep the Fate Point economy going. I'm not sure if that's ideal for a game with a small staff and large player base if the game were to become popular.
Chaosium Basic RPG - I really like this system. The player exposed mechanics are simple, d100 skill resolution with 'training' based increases and a pretty simple to understand D&D standard-ish naming convention. Conflict resolution in the book happens in a 'resistance matrix table' (a simple formula basically), but that's easily coded and interfaced for end-user experience.