The Overwatch beta patch that came out a couple of days ago made some big changes to the game, including the addition of an intriguing new game mode called Weekly Brawls. Similar to Hearthstone's Tavern Brawls, they'll feature unique, and sometimes very unusual, rules that will be rotated on a weekly basis.
Blizzard mentioned two possible examples in the patch announcement—limiting all players to a single class or character, and assigning a random hero on each respawn—but the dataminers at Overpwn have dug up a list of 14 different Brawl modes that are now in the current client.
- Super Shimada Bros.: Welcome to Hanamura. Genji and Hanzo only. Faster abilities, slower ultimates.
- Mystery Heroes: On death you will respawn as a randomly selected hero.
- Justice Rains From Above: Pharah and Mercy take to the skies.
- Arcade: More health! Faster ability and ultimate cooldowns! Faster respawn!
- We're All Soldiers Now: Soldier: 76 takes command of the Control Maps.
- High Noon: It's high noon on Route 66. Headshots with McCree only.
- Show Your Support: All support heroes. Limited to two of the same per team.
- Tanks a Lot: All tank heroes. Limited to two of the same per team.
- Overly Defensive: All defense heroes. Limited to two of the same per team.
- Highly Offensive: All offense heroes. Limited to two of the same per team.
- Heads Up!: McCree, Genji, Hanzo, and Widowmaker. Headshots only.
- The Good, the Bad, and the Ugly: McCree, Reaper, and Roadhog duke it out.
- Moba-Watch: Unique heroes per team. No hero switching allowed.
- Girl Power: Female heroes only. Sorry, boys.
Some of the descriptions are a bit vague, but you get the idea. I've emailed Blizzard to confirm that the list is legit and will update if and when I receive a reply; in the meantime, Overpwn has more info about the new stuff in the patch, including shots of the new skins, screens from the Route 66 map, lists of achievements, and images of the new player border avatars, all of it right here.