I don’t think building something new is going to be a solution. There are already excellent GUI libraries, such as Qt. To become a competitor on the desktop (assuming the desktop still matters), you need a very focused effort to polish some existing desktop to remove the thousands of paper cuts. This is how GNOME 2.x became a very good environment in the mid 2000s. Sun Microsystems invested a lot of power in usability studies, HIG guidelines, and polishing the GNOME UI. Of course, they were motivated to improve GNOME to replace CDE.
Though I am not sure who’d be willing to invest the manpower in such an effort. It will be interesting to see how far the Elementary people get. At least they have the right mindset.