I like the ideas in this thread. I would also add "airtypes". So you can have airport versions in gravel, asphalt or water versions. Each airtype can have a minimum length for its runways and other specs. Also, you can later decide aircraft compatibility with airport types.
I started doing some work to code it but I only work on it during summer.
I converted old airports (layouts) to the new system. In the end, movement of aircraft in old airports and modular ones will be controlled via a new pathfinder. You can also build an old layout and then modify it.
It is something much of us would like to see in OpenTTD, but remember there are people who prefer finite state machines.
Looks real nice, i havent managed to start any code yet as im away on Holiday at present, but from the screenshots that looks like a great start. Is it stable? Can you upload a copy of the diff/win32 build?
Looking at the screenshot though I can see we need to add a transparency layer for those track sections