Ah, Lucioball. Overwatch's Rocket League-esque seasonal mode was designed to be the private domain of the rollerblading, wallrunning Brazilian, for obvious reasons: he's one of the few characters whose moveset makes sense in the format. But according to several videos circulating the net, it's possible to jump in there as Bastion, or D.Va, or indeed any other Overwatch hero who really shouldn't be there.
As the videos below will prove (courtesy of YouTubers , and it is possible to get other heroes into a Lucioball match, but no one seems certain how. All that matters, I guess, is that it's possible, and it's very amusing. Needless to say, Bastion makes for a formidable goalkeeper.
Blizzard has addressed the issue, sadly confirming that it isn't an easter egg, but rather a bug that they'll be looking to fix very soon. To see how the mode looks in action, here's James giving it a burl.