www.makemkv.com

Removing letterboxing/pillarboxing from videos is a good QoL enhancement, especially for anyone watching content in a window on a computer instead of a TV.

h.264 and h.265 (AVC and HEVC respectively) include flags for crop offsets in their respective picture streams. These can be set without losing any picture data. The vast majority of players will also respect these flags, unlike the flags that can be set on the mkv container itself.

here is some documentation on how to do it in ffmpeg.

Doing this after ripping with makemkv is not a particularly huge deal, but it is rather time consuming, especially with 4k movies.

A feature to extract a few frames from the target title so that the needed crop values can be determined are probably a good idea.

Probably not going to see video encoding in MakeMKV. Besides being very slow, it is probably out of scope of what the author intends MakeMKV to be.

Not to mention that some videos have variable aspect ratios, which confuse the hell out of video encoders. And some video uses the letterbox area for subtitles.

Manipulating JUST the header may be simple, but also a lot easier after the video is off the disk.