I installed VS2017 alongside VS2013 recently and the resulting compiled filters were significantly (10-20% or so) slower than VS2013. Just thought I'd mention that in passing.
Anyway, to my point, which is a modest proposal.
I'm developing a filter which necessitates the passing of, for want or a better word, metadata between clips. Traditionally non-video data has been passed around as video in special clips, like MVTools's "super" clips, but this (not necessarily what MVTools does, but other filters doing similar things) is a bit of a bodge. Of course it's necessary because Avisynth has nothing for passing around data otherwise. I believe one of the DG tools uses the low bits of the first few pixels to pass data, for example.
What I'm doing with my new filter (and what I've previously done with
warp) is abusing GetAudio. If it's called with start=1 and count=3, which is extremely unlikely to ever come up in the real world,
and if the passed buffer starts with a special integer, then my code takes over and instead of filling the buffer with audio, it fills it with whatever information the child filter (backwards terminology from Avisynth's, don't get me started on that one) has requested by way of other data in the passed buffer.
By doing this, I can chain commands to add more and more metadata, all of which can be "bubbled up" to the ultimate descendant. Best of all it's completely transparent to VirtualDub2 (my viewer of choice) because all audio is passed through normally, so I only ever have one clip to pass to the actual "doing something" filter at the end of the chain.
But it still has some caveats, namely that audio filters can't be trusted not to destroy my metadata. And if a clip doesn't have audio to start with, it has to be added (and can't be removed until after the final filter runs).
So I was wondering if such a system could be implemented in Avisynth separate to GetAudio. GetData, perhaps. By just passing a pointer to a buffer - or maybe have one other integer parameter as well, for simplicity/flexibility - the system could be made extremely flexible but infinitely adaptable. For a start, custom clip properties would then be easy to implement as a plugin.
As far as I can tell all it would take is adding an extra member function to GenericVideoFilter which, by default, simply passes its parameter(s) down (up?) to the parent (the confusingly-named "child" in GenericVideoFilter), or, if there is no parent, does nothing. Filter writers can then override this to their heart's content.
Anyway, that's just my thought. Maybe it's a terrible idea for reasons which haven't occurred to me.