You’re welcome! 
Category-specific placeholders is a really nice idea - I can see it being especially useful for “Support / Bug” vs “General chat” categories, where the prompts you want users to see are completely different.
Implementation-wise, the composer context usually provides enough signal to do this cleanly:
• New topic: key off the selected categoryId in the composer model
• Reply: derive the category from the topic context
So a possible v1 could look like:
• a theme setting mapping category_id → placeholder list
• fallback to the global rotating_placeholders if no category match is found
If you remember the existing component you’re thinking of, I’d love a link - otherwise I’m happy to add this here as a feature request and gauge interest (and avoid duplicating work if it already exists).
Also - your “ALL funny” list is exactly the kind of chaotic good I had in mind 
(I’d probably still mix in a few serious ones, so the occasional useful reminder sneaks through!)