Halo Wars itself had revealed some upcoming maps, here's some more proof for you. New achievements have popped up on the Xbox.com Halo Wars page in the UK, and they reveal -- wait for it -- new maps. The names of the maps just so happen to be the very same names we discovered back in May: Barrens, Blood River and Memorial Basin.
If you still aren't convinced that new map DLC is on the way, you'll just have to wait for the official announcement. The rest of us will just sit here and act smug for a while.