Edit (9/18/20): I finally made a tutorial on how to do this!
[ Ссылка ]
When I first discovered this free program last year, I was blown away by the results. Even though the program wasn't originally intended for flat 2D animation, I have found it still works quite well, even though there are admittedly still some artifacts.
Note: The program doesn't actually blend the two keyframes together, that was done by generating a result with both keyframes, then crossfading between them in a separate video editing program.
You can download the program at ebsynth.com. I might make a tutorial on how to do this at some point in the near future.
Movies shown (in order):
Robin Hood
The Secret of NIMH
All Dogs Go to Heaven
Bambi
