I think you can get away with it with sci-fi cities. I envision most of them are built by just laying slabs of plasteel down when they need to expand and less about the city naturally expanding. Would also expect most futuristic cities to be walled, so that is one way of covering up the join

Something like this (if you want to convert some of the actual city tiles):
Attachment:
Rough Board Example.JPG [ 50.72 KiB | Viewed 2649 times ]
You could even sand off some of the city features on the city boards and then grass part of them.
A couple of things I am going to experiment with will involve using embossed styrene to make part of the plain board look like the city board and then using filler/sand/flock to show the rural part encroaching on the city.
Given that we are expecting to get these around May, I doubt we will see any official transition boards until the end of the year. At least I can have a go at making some, and if they don't work I will just use the plain boards to make my own set of Realms of Battle boards
Matt