This topic will probably be of interest to you:
If you need bones and rigging then you could have a look at Synfig Studio. There are YouTube videos about it of course.
You can import .mp4 videos using a plugin called Animator Video Reference which will import the frames of an .mp4 file.
This was developed by Scott Petrovic - (known as scottyp here) and can be downloaded from https://github.com/scottpetrovic/animator-video-reference
Instructions/explanations and a url to a YouTube video are included in the download.
If you do import a .mp4 video, every frame becomes a fully painted keyframe so expect to need lots of RAM to store it all.