Left 4 dead 2 DLC next year! http://kotaku.com/5426167/first-left-4-dead-2-add+on-brings-back-bill-francis-zoey--louis Erik
There's always the issue of special infected needing a slot (or do they) and most servers being only 8 slot.
There is unlikely to be a 16 slot server, it will probably be exactly the same as it is now, except half way through the campaign you switch characters or something, but dedi's would be fine for 16 slot servers, just not servers from e-frag and the likes.
Annoying thing with L4D / L4D2 is, the slot sizes change dynamically depending on the game mode. If they do introduce more playable slots, most GSPs will most likely scrap the price for 8 slot L4D Servers and just have a standard price for the new max size.