I would love to see custom tags in Plex, so one can filter for additional criteria.
Here is how I would use a custom tag:
I own a D-Box motion system and I would love to be able to filter for media that D-Box has coded a motion code for. The information could be entered manually or be populated through an add-in that reads an external csv file or accesses an online database.
There are certainly more uses for custom tags, but this would be one of relevance for me!