GTK+ 3.18 Plans For Full Wayland Support, A Scenegraph
For GNOME 3.18 the developers have already decided to go ahead and drop support for Windows XP within GTK+! With GTK+ 3.18 also on the roadmap are CSS nodes support, full Wayland support, a scenegraph/GSK, and a design refresh to the file-chooser.
Still to be improved with the GTK support for Wayland are some client-side decoration features, ensuring all CSS support is there, adding support for symbolin icons, fixing some copy/paste issues, and better handling of window positioning / keyboard layouts / input methods.
The GTK+ scene graph support is a very big action item and at the moment it's not clear if it will be buttoned up for the GTK+ 3.18 release this September.
Those wishing to track the GTK+ road-map can find it at the GNOME Wiki. Other ideas on the roadmap include scale support for GtkImage, touchpad gestures, a new combobox implementation, in-app notifications, a new print dialog, and much more.