I have discovered some areas that are very likely to be used for DLC. Here they are...

An unmarked train tunnel near Nipton. Travel to the Mojave Drive-in and follow the train tracks. The tracks will branch off; one to the left and one to the right. Take the one on the left. You will eventually get to a train tunnel which serves no purpose. There is a boarded up door at the tunnel. There are no enemies here.

And, of course, the Canyon Wreckage. Read about it on it's page.

I will post more possible DLC travel areas when I find them. Please share any that you may have found!