That's what aliases or shell functions are for. Put them into your .bashrc/.zshrc/.whatnotrc and you can have commands like that, without messing up the fine tools you mentioned for the rest of us. Just drop the first space.

ffmpeg--resize-video video.mp4 50%

.bashrc:

    ffmpeg--resize-video() {
       ffmpeg ... "$1"
    }

You'll have added benefit that ffmpeg--<TAB> will suggest you only your own UI, without interference from the tool's own options.