This is a problem that Sabre will have to address. The game is funner IMO with the higher difficulty settings but
friendly fire/teamkilling is a HUGE problem right now.
They need to implement FF on/off options for servers. Also the ability to "lock" down teams-I played with my brother and two AI, then two players spawned into our game and
starting team killing us, taking all the enjoyment out of a good run.
Apparently a patch dropped today(I haven't played yet) that addresses some connectivity issues and such.
As for future content, no roadmap but did find that another free map is coming pretty quick according to this website:
World War Z adds Tokyo map
I think once Sabre sees the sales, they'll probably decide on what if any DLC they want to put out whether it be free or paid.
The game is really fun and a steal at $35. I'd highly recommend it, especially to Left 4 Dead fans!