Most modern buildings (2008+) don't have up/down buttons anymore, instead they require each person to input the exact floor they want to go to on a keypad. It's called "destination dispatch".

This is true on the main landing, but often all the floors above just have simple up/down buttons and you select a floor once inside.

Interestingly my building already require that information before using elevator. I wonder how efficient this building's lift actually becomes.

I was thinking about the problem more from a theoretical standpoint, but you could fit an elevator system with a 'people counter' on each floor.

AI with a camera could give close to perfect information on how many boarded/left on each floor. Maybe it could even identify VIPs like medical personell and give them priority. Of course someone will eventually try to put a subscription model on it.