Gentle reminder to the rules for the feature suggestion forum… please don’t mix up such a mixed bag of suggestions. This way, you’ll have a much harder time to find other users who will vote for the suggestion as they agree with every last element of your post (especially if it’s relatively unstructured) – therefore they’re always worth having their own thread or more precisely to search before posting them to avoid duplicates.
As for your main suggestion
There’s an existing suggestion to match items based on a (sound) fingerprint. This is kind of the same approach you suggest – with the exception that they’re looking at matching the media this way… which implies it can also identify that items are the same / similar.
I’m inclined to consider this a duplicate.
As for your “other minor requests that are not worth separate threads”
List items that failed to scan
Plex won’t ignore files because of their content… you can even feed an empty text file to the scanner, name it like a movie file (incl. the related file extension) and that file will show up in Plex. Also, there’s already an existing suggestion asking for a report/overview within Plex to highlight such issues
→ Duplicate
Library filter showing missing items from collections
There’s an existing feature suggestion aiming at getting an overview of items missing in a collection (or even displaying them in some way as placeholders inside the collection.
→ Duplicate
Being able to split again in nested directories
Not exactly sure what this is about. Seems to be working for me… but I might be “holding it wrong” 
Tell Plex which directory contains which season
There’s an two existing feature suggestions which already address this:
- Settings per folder under TV Shows → correction on folder level (as per your description)
- A possibility to correct a wrong season match → split/match based approach to fix bad assignments.
Include comments
There’s an existing feature suggestion asking for “curator notes” – basically a short message of what you might want to share with users about that movie, e.g. a warning as per your example.
→ Duplicate
Subtitle handling
That’s a major one with as many existing suggestions on how to improve/change/re-do the handling of languages.
The most prominent one is a suggestion to e.g. set subtitles for an entire show but there’s also ones that simply ask to pick up the previous subtitle selection during auto-play (e.g. for a series).
→ Lots of duplicates / similar suggestions, though not for the specific use case of “specifying subtitles for all users”. That being said, that’s an unlikely feature suggestion to be implemented given it’s based on the assumptions of users blindly adding media w/ bad/unstructured/untagged/… subtitles and some wide mass sharing that’s not on Plex’ radar.
TL;DR:
If you feel the suggested duplicate for your main suggestion does not cover your suggestion I’m ok to keep it open – focusing on only that part. Otherwise I’ll close this suggestion.
