It would be nice to have an ability to fully/partially override the UI for a custom game page with an access to a steam id of a player who opens it.
This would allow the game which have persistence to show player data without entering the game.
This would allow better out of the game tutorials, interactive and/or with sounds/videos.
This would allow implementing a custom content/news delivery system which the current system does not offer. Big changes and news have to be shoved into the game or onto a foreign website. Not cool.
If coupled with changes like adding spectator slots to custom game lobbies this could power interactive elements for possible matchmaking, for example it could allow selecting a character before the game starts and making a custom "play" button which would make a HTTP request to a bot which would set up a special lobby and invite players there. A matchmaking of sorts. Awesome.
It would be nice to have an ability to fully/partially override the UI for a custom game page with an access to a steam id of a player who opens it.