I saw this billboard on Antelope Valley and I don't recall seeing it before... since most in-game ads are related to something (often Dovetail-related) I wondered if it might be a hint for a forthcoming route - maybe something that Union Workshop is working on?
Let them take their time and not rush things. Union Workshop have a history of releasing good and beautiful scenic routes, I think they only had to issue one patch but that was due to an asset pack that was duplicate and caused the game to crash. I found this out, contacted UW's T9 (that's his nick) per email and the issue was quickly fixed with a Steam patch.
What's the plot of "Hidden"? I suggest something about a sumurai and his daughter, who avenges him after he was falsly accused from an unknown person and forced into suicied so the deceiving persons are hidden.
What about the poster of NYC on ECML. Surely that means something. Or the White Bird Travel poster at Doncaster. That is definitely hinting.