Xfce 4.12 is due to be released in March and early on there was talk of Xfce 4.12 using GTK+3 as it's toolkit rather than GTK2. However, it was ultimately decided that a full conversion to GTK3 won't happen for the 4.12 release.
What has been happening to Xfce 4.12 is slowly transitioning to the newer version of the tool-kit using by GNOME. Xfce 4.12 is aiming to be GSEAL compliant (a compiler macro for pointing out direct access to object fields rather than using the accessor functions, as recommended by GTK3) and to stop using custom/deprecated GTK widgets. Using the appropriate functions and stop relying upon old functionality is a major step towards ultimately migrating to GTK3, which may happen in Xfce 4.12's follow-on release.
Xfce 4.12 also aims to partly support GTK3 by allowing the parallel building of GTK2 and GTK3 libraries for libxfce4ui and Exo. Core components will also be allowed to optionally compile against GTK3 for testing purposes. GTK2 though will remain the official toolkit target for Xfce 4.12. (The GTK+3 migration in libxfce4ui is described as being about 60% complete, as of September when the Wiki page was last updated.)
Aside from the slow toolkit transition, there's improvements for end-users to be found with Xfce 4.12. The Thunar file-manager in Xfce 4.12 (Thunar 1.6) supports for changing multiple file properties when selecting multiple files, invert selection, drops user directories, improves in volume (including mounted remote locations) handling, improved shortcuts in the side pane, and tabs are now finally supported by Thunar.
Other changes to Xfce 4.12 include API clean-ups within libxfce4util and improved multi-head configuration with basic support for an extended desktop mode.
Xfce 4.12pre1 is expected to happen this coming sunday (27 January) along with entering the feature freeze and soft string freeze for this next major release. Xfce 4.12pre2 is expected in mid-February, Xfce 4.12pre3 in late February, and then the Xfce 4.12 final release around the middle of March.
More details on Xfce 4.12 can be found out from the Xfce Wiki.