We're thinking the same thing, cameni. While crash detection will be desirable, the main thing will be some sort of interplay as to when the aircraft is on a runway. Collision detection will have to remain within Outerra, but there's bound to be situations where the X-Plane terrain would actually be higher, and that would be like flying into an invisible mountain.
The key gain from the merge of Outerra and X-Plane would be X-Plane's flight dynamics and systems modeling. Given that both would have to be running at the same time, it might be necessary to run the merger on two computers.
The main solution I see is to use the Outerra world and to modify the X-Plane Global scenery so that the whole planet is nothing but water. Without terrain, the airports still exist - just at sea level. So while the aircraft is on the ground, the Outerra physics engine would rule. X-Plane flight dynamics would only apply after the aircraft took off.
While there may be a way to integrate the two into a comfortable desktop experience, my aim is more to support the full size cockpit community, mainly people running more than one computer to satisfy their flight sim yen.