A Game War!
This is not focused around users, but around the various RPGs we have: 13th-age, dungeons-and-dragons, fate-core, dresden-files, etc. We determine, fairly tongue-in-cheek because the entire premise is a little silly, which is the greatest RPG ever, based on things like question/answer activity within the competition period.
It's silly and lighthearted, and the competition itself should run with the idea that it's silly. The writing about the competition should not take itself too seriously, e.g.: "YOU determine which will be the greatest RPG, by these completely arbitrary and actually meaningless criteria!!"
We could display progress in various ways, such as meters showing the various system tags' relative scores, or a battleground with coloured soldiers being more abundant for the winners.
This isn't necessarily going to be a fair competition. Some games like Fate have a fairly simple ruleset that won't necessarily have many questions (then again, maybe this will prompt them). Other games have fairly complex and often convoluted rules which clash in all sorts of ways (no prizes for guessing which popular RPG I'm thinking of) which already give people a lot of opportunity to ask questions. There could be things done about this: games with low current activity score points more easily, etc. Remember ultimately this competition shouldn't be taken seriously, nor taking itself too seriously.