I see applications moving to gtk3 from gtk2. It takes time, others are rejecting movement. LXDE and Xfce 4.10 will stay at gtk2 for a while, but for example VTE and gnumeric/goffice alerady gtk3 based. For sure if you want to use latest releases we need a mixed environment for na lng time or stick with gtk2.
I think gtk2/gtk3 libs can coexists so it may work on a runtime level of gtk itself. On the other hand a whole bunch of libs must be there also for gtk3 like pygtk, ... Not an easy situation. But experiment can be done with few packages with minimal gtk dependency.
Any idea?