OK, so I know that this is far outside of Reaper’s normal use however, we already have plenty of tools in Reaper for stretching audio, right? BUT when it comes to what to do when slowing down video connected to that audio, it’s always been a different story.
Enter Ai video interpolation.
RIFE is the current fastest and best one that I know of, found on GitHub here:
https://github.com/hzwer/arXiv2020-RIFE
Older (slower) Ai algorithms are DAIN and CAIN
Comparison here:
https://duckduckgo.com/?q=DAIN+vs+RIFE&t=brave&ia=web
Now, I’m not sure if these could be implemented as a render option, and right now as far as I know, no one has implemented these into a software package for anything other than a multiplier of frame rate however, if Reaper had the ability built in to export interpolated video with the same ease that audio gets stretched, ie on a sine curve or similarly variably-adjustable timeline, it would be a massive industry-first.