I think you should add some zdoom functionality to vavoom, like doing slopes the way zdoom does it, pathnodes for monsters, cameras etc, etc
This could encourage map/modmakers to use Vavoom
Also it would be great to implement 'Unrealistic' actor classes (they're easy to deal with, and powerful - from the viewpoint of a modmaker), portal surfaces (like Unreal 1 map DM-Eliza ( ??? or something like that)), mirrors (like zdoom, but there is a way to do it on thin walls in GL), and reflective surfaces (like ZanZan TC's first level floor in ZDoomGL)