Serious thought just occurred to me- what would L4D3 look like? Barring the story and setting, given the massive success of valve's free to play model, would it be free, with community cosmetics? Or more of a CS:GO weapon skins and maps type thing? Or will it be on it's own, like L4D2? Will there be customization? Easy mods? This will* kinda be Valve's first single player game in the post-F2P era.
*Assuming its a thing, which most signs point to "yes."