I have two at the moment.
Suggestion 1: Forum categorization ideas: Rename the general discussion category to Non-Pony discussion. In my opinion, the term general doesn’t properly express the category’s intent.
Add a new category called something along the lines of ‘Forum Fun’ for threads like
You’re Banned,
We Can Only Talk In Pictures,
The Word, etc. When making threads like these, it can be very hard figure out/know what category to put them in, so a category for them to belong would be nice.
A Pony discussion category would be nice too, as the episode discussion category is rather limited in scope.
Edit: I think an uploader(or whatever you what to call it) thread category might be worth thinking about. There are plenty of pony art in places other then derpibooru, so being able to communicate with other uploaders without threads getting buried could be useful.
Suggestion 2: A tagging system for forum threads. Unlike the image tagging system, this one could be mainly used to detail the rules of certain threads.
For example: tags like Safe, Adult, NSFW, etc. could be used to convey rules, while tags such as Forum Game, Character Appreciation, Theory, etc. could be used to allow users to more easily find threads or check if threads already exist to prevent duplicate forums.