Thinking of another mash-up, putting together two of my favorite games, Dungeon World and Legend of the Five Rings. My current DW / HF PBP is stalled, and my face to face group is about half way through the campaign I'm running, so in the new year, I might do some work on running L5R using the DW rules.
Some parts will be easy to deal with - for stats, use Earth, Air, Fire, Water and Void. In place of races, use clans (or families), and for classes, use Bushi, Shugenja, Courtier, Monk etc at a basic level, or in a more specialized form, use eg Crane Duelist. It would a considerable amount of work though. Alternative paths could become advanced moves, and the advanced classes could be compendium classes.
It'll take quite a bit of work, but it looks promising.