So the Overwatch community has a love/hate relationship with shipping of any kind but the dissenters are usually outnumbered by the vastly more talented people who dress up, take photos, and draw fanart, so it's still pretty easy to enjoy what you want.
This particular game with its attempts at rich back story (that is mostly forgotten once you have anything to do with actually playing it) doesn't really have any canon pairings. You could frankly throw a few darts at the hero select screen and come up with a pairing that will have as much canonical backing as any other... not that the internet hasn't tried.
Of course, this makes the game particularly conducive to large group cosplays, or even smaller threes or twos. Soldier 76 and Mercy as seen above are a popular pair, first because of character, but also because they are two of the most commonly picked characters in the game. You can also find the officially licensed costumes and props available through Spirit Halloween (along with Reaper and D.Va) if you're looking for something quick, easy, but undeniably pricey.
(image source: ACParadise)