Template Folder Changes
As of version 5.0.0-rc2 (commit be0978f3 to be precise), there was a change made to the way the template path is determined. The ambiguous environment variable KICAD_PTEMPLATES has been deprecated and replaced by KICAD_TEMPLATE_DIR which by default should point to the template folder installed with KiCad. This path may not always be correct depending upon how KiCad was installed. At least it can be configured by the user if the default path is incorrect.
Mask Clearance Generation Changes
As of version 5.0.0-rc2 (commit 93c7f65d to be precise), there is a change in way the solder mask and solder paste clearances are determined. For pads defined with copper layers, the solder mask and solder paste clearance calculations remain the same. For pads defined with no copper layers (referred to as aperture pads), the inherited clearance and ratio settings are no longer applied and the pad is plotted as defined.