How are the starting positions in Lúcioball chosen

Really, the game only seems to care if you're on Attack or Defense. On KOTH maps, both teams are Attack.

The warnings I remember seeing are:

For both Attack and Defend:

• No Damage Dealers
• No Supports
• No Tanks
• Too Many Tanks
• Too Many Supports
• Too Many Of The Same Hero

The "Too Many" ones appear if there are 3 or more.

For Attack:

• Few Damage Dealers
  • If you only have one Offense.

For Defense:

• No Defense
• Few Defense
  • If you only have one Defense.
• No Builders
  • If you have no Torbjörn or Symmetra
• No Snipers
  • If you have no Widowmaker or Hanzo

Anyway, what this really boils down to...

• Attack
  • 2 Offense
  • 1 Tank
  • 1 Support
  • The rest filled as appropriate, but keep in mind the Too Many rules
• Defense
  • Widowmaker or Hanzo
  • Torbjörn or Symmetra
  • 1 more Defense if Symmetra is picked above.
  • 1 Tank
  • 1 Support
  • The rest filled as appropriate.

From an interview with Michael Chu, a Senior Game Designer on Overwatch (emphasis mine):

If you look at a game like World Of Warcraft, the Horde/Alliance split is such a big deal and such a defining aspect of the game. But Overwatch, pretty much from the start, we didn’t want to restrict ourselves that way. Because the game is so much about having the flexibility to play all these heroes and finding all the ways they play well together or against each other. Because of that we decided that the gameplay of the Overwatch universe wouldn’t be bound by that.

Our story justification – what I usually tell people – is that I think of it as, it’s clearly not canonical, but I think of it as how in comic book stories sometimes Iron Man and Captain America fight each other. Maybe in this instance on this map they’re fighting – for some reason they have some ideological difference or something that’s bringing them against each other.

But I think ultimately it was important for the game that you were able to play whatever characters you want. Blizzard has this development philosophy which is ‘Gameplay first’ and this is definitely an example of where we’ve gone with that.