The 3rd DLC and 4th is called Halo CEA! 20 dollars worth of map and a 5 dollars game all for only 40$+Tax! :D
(Halo CEA comes with 6 matchmaking remakes and 1 firefight map)
All i know about future maps is that when you Buy CEA you get a voucher to download 7 maps to Reach, 6 MP maps, 1 Firefight). I was hoping for a 3rd DLC though. Buy you never know, maybe we'll get some news soon.