One of the holy grails in development is a universal GUI library, much like the quest for the one equation that explains life, the universe, and everything. Unfortunately as it currently stands such a library is further away than ever before.
A project that wanted to succeed is the BoostGUI library. Unfortunately it has never got out of the research stage, let alone the planning stage. It was envisioned to be the Boost Library addition to make GUI development at least have one standard. The research page (click on the BoostGUI link) though provides great avenues for people wanting to know more about GUI development, some of the conversations on mailing lists that the wiki links to are excellent and thought-provoking.
I don't know how all of this will work in ten years time. It was certainly an admirable aim, but with technologies like XUL getting more traction and getting more mature, the answer may come from a completely different direction to what all of these toolkits are aiming to do. They'll provide a simple cross-platform framework that will get added to over time and eventually have the richness of user interface abilities required by heavier applications. Certainly I don't think these types of libraries can be designed from scratch to be everything, and they have to grow, whereas most GUI toolkits have gaping holes in "to be implemented" areas, whereas the simpler domains mean you can easily get 100% coverage and then add more.
I can't believe this, over the past few weeks I have been talking myself out of heavyweight GUI toolkits... Scary stuff. Don't get me wrong these toolkits are still my primary interest for any GUI development, but I wonder what the future could hold as web development has really started to spped up. We all need not only to think of this new multi-core future but also the web-based platform future.