Weigh all passengers anonymously to optimize fuel load for best performance.

Track meal consumption on connecting flights to adjust service timing, avoiding back-to-back meals for better passenger comfort.

Using hydrogen tanks in the rear of the plane, for example. This could work on shorter routes such as domestic flights or intra-Europe.

Include meal information as early as possible in-app.
This is good for people with allergies, so they abstain from it.

More variation in aircraft sizes = optimized cabin factor = less fuel consumption/passenger

Giving an incentive and getting more passengers to travel light should reduce aircraft loading and fuel consumption. Alternatively allow more cargo.