It just needs Snapchat. Throughout my time with windows phone since the wp7, the condescending android users would always hop in here and mention that one specific app that wp needs to survive. It started with Facebook, then that came, then twitter but that came too, then instagram, then Youtube, and a slew of third parties came and then it became available on browser and fanboys kept themselves quiet, and now the "it" app right now apparently is Snapchat, and Snapchat seems to be the holy grail of all apps. So if you were to bring one app, I'd ask for Snapchat. So the condescending wp haters can pick another app to nag.
Maybe the next on their list is Grindr.
Everything can already be accessed thru the browser anyway.