Support for third-party campaigns, as well as tweaks to the skill-based matchmaking system, are also in order. nope Valve further notes that the Left 4 Dead software development kit, or SDK, will be released shortly after the April 21 debut of the Survival Pack update.
Valve's Zoid Kirsch elaborates:
To support the SDK, we are extending the matchmaking system to allow for third party campaigns. We will be adding extra features so players will be able to find dedicated servers running the campaigns they have installed. They will also be able to see friends playing third party campaigns, and be directed to download them as necessary. Another major feature we are planning to add to the matchmaking system at a future date is team-based Versus matching. This will allow two teams of four to locate each other and compete in a Versus match.Thanks to TnS_hun for the tip.
We are also revamping our player matching based on skill level. Currently the system tries to match people with the same skill level and best pings together. We will be revisiting this system to tweak it based on data we have collected.