I'm unlikely to add anything new to VVVVVV at this point (it's six and a half years old now!), and any updates I do in future will just be to fix compatibility with new operating systems, and things like that. Adding new features to custom levels (e.g. custom PNG minimaps, tower levels, fixing behaviours like how cutscene bars work) would branch the game into an old custom map format and a new custom map format, creating compatibility issues, which I really don't want to do!
What I *could* see happening are additions to the map editor that don't break the current map format - like, it'd be cool if there was a switch to allow internal scripting that automatically took care of implementation in the background. That sorta thing!