Well I guess the airports should be done in a "normal" way, making models for the buildings and all, except for the runways that are to be modeled directly in engine since they are using special data structures optimized for seamless integration into the terrain. There will be importer for the models, you should be able to convert from Collada format. There will probably be also an importer for the roads and runways, to get them all in, but we have to make an interface/SDK for it, so that skilled people can feed the data from their sources.
Another path is importing OSM data. OSM has a huge database (150 GB uncompressed XML), and for an effective import we'd need to partition the vector records into tiles that roughly cover terrain tiles in our projection, for the importer to process it effectively.
There are also other types of data in OSM that could be used. Ultimately we'd like to feed city/building generator with the polygons defining the city parts. The effectiveness of OSM importer is crucial here.