Similarly, go to the Tales of Symphonia board and make a topic about who Lloyd would be with (Lloyd x any character who appeared in the game).
The contests already lead to some contention given they're basically popularity contests. Add upsets, technical problems or vote stuffing to make the results questionable, or indications of outside tampering (previously from Battle.net or Nintendo fansites, but now 4chan and Reddit) and the board that discusses them will look like a warzone.
The mere mention of a certain girl whose name refers to a three dimensional container with six sides warrants your being perma-banned from the internet, and the planet. This statement may have been enough...
Kate Crate? Countess Winifred Shipping-Container? Boxing Helena? Oh wait, that last one's a movie title.
Here's something that you might want to try out. Look for a forum (preferably showbiz-related) that has a fairly large Filipino membership. Create a thread titled "Kapamilya or Kapuso?" Talk about network ratings, and watch Armageddon unfold right before your eyes.
A little broad to fit anywhere else, but digital piracy as a whole. There's a reason the page pertaining to the issue has been locked out of real life examples.
Can sexual or violent genres of literature, anime, Western Animation, etc. have a bad influence on the development of young children or not? It's best not to ask this question on any social network; it's as good as declaring war.
"Bad girl" art: harmless erotic fun, or disgusting sexist/misogyny. Keep in mind female artists tend to draw in this style as well (for example Amy Matthews) adding another dimension to the debate.
Can get this way among some martial artists, many of whom believe that their fighting style is undefeatable and anything else is just a money making scam.
Go to a police forum, or ask a group of officers,what they think of reserve police. Then stand back and watch the uproar.
Missing person cases can get kinda contentious if you believe that. Mostly over how the police handled the case, or whether or not can you put some of the blame on the victim with out being criticized for victim blaming.
On tumblr there was a great thought provoking blog called Black People Confessions. It featured confessions from black people from all around the world who would discuss things that affected them as black people. Some confessions was sad and others was humorous, but no matter what it was always respectful. But once it became popular? ....ooh boy the flames was hot. The blog devolved into heated BITTER gender wars, and intense flame wars over colorism, and "light skin privlege". And then there was the interracial dating debates, my GOD the interracial dating debates!!. The moderators had to eventually put a temporary ban on those topics cause it got so bitter and passionate.