One option would be to have people check in, drop off their luggage, and even go through security in some convenient location in the city center and then take a high-speed train "inside security" to the gate. (Maybe you could even have trains to two different fields.)
The Madrid airport offered this service, but it wasn't very popular, nor widely known. You checked in your luggage downtown and hopped on the subway to get to the airport with just your carry-on. I can't find any reference now, so it must have been discontinued.
You can do something fairly similar in Japan. They have luggage shipping services that are quite cheap and reliable, and have some days of storage built in. So you can take a train between cities without carrying everything, or maybe skip your big luggage at one city in your itinerary and have it at your hotel in the next city. You could also deliver it to the airport, but you have to build in some hours of lead time.
God yeah it's like a hypothetical version of the AirTrain that isn't a huge pain. Last time I flew out of JFK from Manhattan IIRC the easiest way was to do the E or LIRR from Penn to the AirTrain anyway, so might as well streamline the whole shebang.
30 years ago that was an option in Almaty (was extremely convenient for me, as we lived literally 2 block from there). You could check in, drop baggage, go through security and then ride on a bus directly to the plane. Same in Moscow, not sure about other cities (did not fly to other locations back then).
However, this was only partial solution, as it worked for departures only, not arrivals.
