I could see Valve controlling a bit of a monopoly in the game launcher and gaming social media markets.

A pro-consumer change that the EU could impose would be to split up the game marketplace from the game launcher and gaming social media markets through intercompatible APIs.

Maybe you could download games from steam in GOG or Lutris, and the steam overlay works on GOG or Lutris too. Maybe your discord friends could show up in the Steam friend list.