Maybe they can start a kickstarter campaign to get funds for it. I'm pretty sure that a better UI would attract a lot of users that now hesitate between using Photoshop or GIMP.
For preexisting OSS, I think it is difficult to get a successful Kickstarter campaign because they cannot really sell something for pledges. Have there been any successful Kickstarter FOSS projects ?
I think Bountysource is better here but for mega issues like UI redesign or GTK3 port etc a campaign may be the right way to go.
Turns GIMP developers encourage private campaigns focussed on developing specific features.
Another good article on current state of GIMP: http://libregraphicsworld.org/blog/entry/gimp-is-20-years-ol...
http://blog.en.uptodown.com/gimp-20-years/