Hi guys. Figured this might interest you here.
About 2.5 years ago I started doing a full disassembly of Super Mario Bros. 3, and about 6 months ago, I finally finished! (It was on and off again as time and interest allowed.)
But before I went ahead and released it, I wanted to have a level editor that directly supported it and be the first one to make something with it, so here is the beginning of what I'm working on. It's currently called "Super Mario Bros. 3Mix" and sort of works on taking things from other Mario games and mixing them backward (or forward) into SMB3.
Currently released is a Two World Demo which basically runs something of a "summary" of SMB1 and SMB2 (USA) while it starts kind of transitioning into SMB3. Future worlds will probably be even more imaginative.
Download, information, and preview video available at: http://sonicepoch.com/sm3mix/
I plan on updating that website "soon" with the disassembly source (I won't make anyone wait until I'm done with 3Mix, whenever that might be) and some other stuff. Check back there for updates and in the meantime, please give this a roll.
Disassembly and documentation is now uploaded. Let me know if anything is really wrong with it; I spent 9 hours on a Saturday on it and just wanted to get it up there, so something might be wrong. Just let me know.