R15 had a serious issue that prevented the Avisynth compatibility stuff from working with anything except YV12, this release mainly fixes it. There isn’t much else new of interest in this release.
If you need more stuff to fidget with then go try the new resizers for VapourSynth too. They will most likely replace swscale once they become stable so testing is appreciated. They’re based on the dither tools code.
Next on my own todo list is a plugin to evaluate expressions at run-time, think mt_lutxy(z) but without insane memory requirements for 16bit formats. The other thing on the list mostly is about VSFS and that is to improve error handling and adapt the code to the latest Pismo File Mount API.