Also, I assume it would be for lemmy as a whole and not just my instance right?
I think having an option for posts to have tags would be neat and make it easier to find more fitting posts since the title won't always have the word you're looking for.
I love the idea. I can see how it could be abused, but that's true of anything when you get a group of humans together. I think your moderation idea could work.
I think I have generally seen such things put here. Apologies as I have not yet successfully made a good link yet. It looks like somebody has spammed a few messages, but if you scroll down you will see discussion about Lemmy and its development.