Read the metadata from mkv files like you do for mp4 / m4v.
Yes, you have online metadata. Yes, I can add it to TVDB / TMDB (and I do). But still, why even bother having a ‘prefer local metadata’ option if you aren’t actually going to support local metadata in common file formats?
There’s already an existing suggestion discussing an option for Plex to read embedded metadata from MKV files. I suggest you comment/vote in that thread in order to help us avoid distracting or cannibalizing votes. Unless I’m missing some unique aspect of your suggestion I’ll close this thread as a duplicate.