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.
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...
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.
Diaspora was successful in the sense that it got lots of money. It was slightly less successful in delivering on its promises.
https://www.kickstarter.com/projects/krita/krita-free-paint-...
https://www.kickstarter.com/projects/krita/krita-open-source...
Though the reasons it worked for us don't really apply to the Gimp situation.
But the platform selection was probably wrong - a lot of people probably was distracted by it.
Note that he's not core team member. It was a personal fundraiser.