Valve should totally start adding cosmetics to L4D2 along with a workshop.
Since users are taking this thread too seriously I'll provide pros and cons below as to why I personally believe a workshop plus community made content in game would be a good idea.
-Gets community modders more involved
-Allows more customizability in game
-Gives more reason to continue playing after you've beaten all stock maps (Acquiring new items)
-Custom content is always fun
-if trading / traded items were included users may occasionally harass users in game to trade as well as waste time (There is still votekick)
-Some users may not like updating their game occasionally just for cosmetic items
-Users may think they need to buy all cosmetic items added in game because if they do not they feel as though they do not have the complete game
That's about it really. I believe a workshop + a l4d2 store would help the game more then hurt it and I can't really think of any serious cons to add that can't be solved by users.
Note: This thread is in no way related to or inspired at all by TF2. If you believe this will ruin the game look at Portal 2 for example and how that turned out. It was barely used and did not negatively impact the game at all.