One of the most annoying things about flying is that airports are generally a long way out of town. This means that you have to leave extra early in case of bad traffic on the way to the airport, and it means that upon arriving you still have to get a taxi/bus/shuttle to the place you actually want to go to.
This seems to be an inevitable consequence of aeroplane technology - it's very loud and smelly and requires a lot of space for take off and landing. But if cities and aeroplanes were designed from scratch with each other in mind, one could imagine that this could be different. While take-offs and landings would still have to happen a way out of town, perhaps boarding and deplaning (what a silly word - what's wrong with "disembarking"?) could happen in the middle of the city and then the plane could taxi all the way to the runway. Perhaps pieces of the plane essential for flying but not for passenger comfort (like wings and tails) could be made detachable - they would be attached right before takeoff and detached right after landing. The rest of the plane could then fit into a tunnel for taxiing into the city. Sounds a bit dangerous, perhaps, but I'm sure there's a way it could be done without compromising safety.
Of course, because this would require massive infrastructure investments simultaneously all over the world it's unlikely to happen for a very long time (if ever) - probably not before our current idea of air travel is superceded by something altogether cheaper, faster, more comfortable and more environmentally friendly anyway. So I guess it will remain just another of my more crazy ideas.