If you ask me, Frontier are sitting on a few Flat rides that they could have easily introduced in 'World Trade' but are instead holding them back for the next DLC which will be a Rides only pack.
Take the last couple of updates, the 1.6/1.7 ....we got Hotels, and Recolourable Staff, the1.8 we got Restaurants (same thing) and Recolourable Ride Attendants (again, same thing) all created at the same time IMO, but they held onto them to make each free update appear more fuller in content, same reasoning why no flats in 1.8/DLC.