FFMPEG - Interpolate frames or add motion blur

You can try

ffmpeg -i source.mp4 -filter:v tblend -r 120 result.mp4

or this from https://superuser.com/users/114058/mulvya

ffmpeg -i source.mp4 -filter:v minterpolate -r 120 result.mp4

There are filter for motion blur

You can use Butterflow as it uses ffmpeg https://github.com/dthpham/butterflow

It's a command-line tool that can:

Increase a video's frame rate by rendering new frames based on motion (pixel-warping + blending). Make smooth motion videos (simple blending between frames). Leverage new frames/increase in frame rates to make fluid slow motion videos.