[User feedback required] customized container muxing support?

Is it possible to generate mp4 file (through ffmpeg?) with VP9 as video track and AAC as audio track?

What I get with JD:
mp4 => v:h264, a:aac
webm => v:VP9, a:Opus

Currently, I manually mux VP9 from webm and AAC from mp4 to get a new file: mp4/mkv.

Can this be simplified?

[Jiaz]

Please provide example links and contact us via support@jdownloader.org