As far as I understand epub (all versions) is already modular. It says in the spec that if the device can't support whatever function then it can gracefully degrade...eg. if it's an e-ink then it isn't expected to produce different colors.
Likewise on epub3 if a device can't support video (e-ink again...hmmmm...yup definitely won't buy another e-ink
) then it simply doesn't play it...the key is that it should not crash the device or fail to load the book.
A standard simply means that IF you are going to support the functionality THEN you need to support it in the following standardized manner. This provides consistent results across platforms; results that developers can depend on when they are creating the books.
So, IMHO, there really should be no reason not to release epub3 (with some fixed items) which include all of the video/audio/mathml/css3/html5 etc. David's Cartoon is humorous, but letting each company come up with their own version of epub would make matters far worse. It is much better to have portions of a standard that you don't support than creating your own version of a standard.
Cheers!