Basically, instead of choosing from a preset of 9v9, 6v6, 4v4, etc, you could choose "custom".
You would be prompted with a window that allows you to add classes, and it would generate a server config based on what you chose.
So you could choose to set up a lobby with 9 spies on each team, or make a game with 5 demoman on each team. How about scout, soldier, demo, medic, sniper, spy? What about just 4 medics vs 4 medics?
This also allows for new forms of gameplay/gamemodes to emerge. Who knows what'll become popular.
The only downside to this is that there would be an increase in "joke" lobbies.