Gamifying the editing of other people's posts and editing semantics of content after-the-fact
Yeahhhhhh no, I think you just suggested StackOverflow without taking even a look at the recent or not recent discussions on how it behaves (eg.: the moderator strike) or why.
I think there are ways this could be kept in check, but basically you'd be providing an incentive towards just going around and adding tags for content nilly-willy, potentially leading to something very similar to SO's "closed as duplicate" (duplicate is on a completely different language / software stack / problem domain) issue.