Basically, the ability for a logged-in user (or maybe logged out, using cookies/user storage) to be able to mark that they have finished the game, also probably other status tags like "in progress" or "currently at end of content". Also, probably a way to filter out, say, complete games in search
(edit to add - tagging would be only visible to the user, unless you also want to add in opt-in public listing or something)