Wet blanket time, folks!

Wonderful idea, loads of potential, agreed. But I think you may need to read some of the other threads in here about music notation software to remind yourself just what sort of hurdles we are talking about here. There are plenty of software products that can already easily distribute notation and sound at a nominal cost - such as Melody assistant that costs about $15 to buy and then can display notation you have prepared on a web site at no extra cost. Or Finale Notepad, that can do it for free, providing your music is limited enough. Virtually any music software can be persuaded to produce a GIF or similar issue, or you could install a PDF creator and print to it then attach PDF files alongside the MIDI. There is not really much benefit in producing a new standard if the near-free alternatives are not being used.

More importantly, notation doesn't actually represent the sound. Variations between verses, for example, or personal embellishments get removed most of the time, otherwise the notation becomes too unwieldy. There are pragmatic problems of notation that simply do not arise with audio, such as the US and UK using different paper sizes which in its turn alters the best way to lay the melody out in terms or bars, repeats and breaks, to make it as playable as possible. In computer terms, you have an extreme form of what Jackson Programming Methodolgy called 'a structure clash'

You may also want to do some backgound research on NIFF, which was an "agreed" notation-interchange standard before you get too far down the road on such a project.

After all those caveats and depressing sentiments, go do it! If it can be achieved, everyone will be enternally grateful - especially me! I have, at the last count, 6 incompatible music notation programs installed on my PC.