We'd be happy to have contributors to the new official theme. This is the reason of my opening this thread. :-)
You are welcome to check the master branch and propose some of your icons (or newly made) for the missing icons, or even propose different icons for the existing symbolic ones if you believe you have a better symbol to represent a function. Just make bug reports with your proposed icons.
We had a problem (or at least an uncertainty) about the CC by-sa license. If I remember correctly, this is because we are embedding icons within the main code. Well at least for what we choose to be the default icon theme, but since we may want to make the symbolic theme the default theme at some point (though I am personally not set on this), we don't want to block our future choices.
So that means you'd have to relicense as GPLv3. But since you say that's no problem below, well that's good.
See the FSF answer to our question about GPLv3 - CC by-sa compatibility here: https://mail.gnome.org/archives/gimp-developer-list/2013-October/msg00112.html
Using SVG icons directl would be very interesting, in my opinion to handle dynamically many different kind of screens.
If all platforms are not perfectly handled though, I guess this will wait until all main platforms are fully working with vectorial icons.
I'm not sure we should have too many themes. A dark and a light seems a good deal to me. If people wants more choices, they can always search and download (or make their own). The goal is not to suit each and every one's need, because it is impossible (there is always someone who is not happy with the default). So we should just propose a few sane choices, and let people with more specific needs install other themes.
This is how it works in GIMP 2.9 (and 2.10). Old themes with icons set in the gtkrc simply won't work, I believe.
Checking the page, it does not seem you are the author of the icons (you are citing various people). Are you? If you are not, or not the only one, you'd have to ask permission to everyone who participated for the license change. You cannot do it on your own (if that is your case).