Basically the entire library was cleaned up in 4.8, so it doesn't need any big work this release.
Right now the preferred application helper installs a set of 'known' applications so certain cagagories. This works fine-ish, but if the user for example uses a web browser that is not in this set, he/she needs to manually set it. Instead it should look for installed desktop files that provides a special mimetype or category.
Exo had a small support library for widget in libxfcegui4. The idea behind this was to keep it around until libxfce4ui was born. This library is around since 4.8, so now it is used in exo as well (libxfce4ui is a new exo dependency!).