There are possibly some developer plans for Steamworks integration already, but I'd decided I'd write down some features I personally would find useful if implemented:
1.Linking of Steam profiles to the "Players" screen ingame. Namely, a clickable link to the Steam profile.
2.Steam invite to a game.
3.List of last 10/20/30 Steam players you've played in multiplayer with. Useful if e.g. you've just finished playing on some server and want to establish contact with someone you've just played with.