This is not a negative comment, just an observation. Air Berlin used to hub many flights through PMI & Berlin. If you wanted to go from northern Europe to Canaries you had to hub through PMI. It took forever on their schedule and involved getting off/on etc. If you could drive a couple of hours to another airport and fly direct with another operator which would you choose? It took all day to go anywhere. The idea might sound feasible and perhaps attractive, but with so much choice of carriers, and where convenience is worth paying for, it might not work as well as expected.
It strikes me as a tourist thing, not business. If by shifting my flight by a day or two, and driving to another airport I can fly direct at reasonable cost then that's for me. I can't see how 2 sectors with associated charges can be cheaper than one.