Pardon me if this has been suggested elsewhere but my search didn't bring this up.
It would be neat to have actual construction vehicles to accompany contractors for larger projects. Example when laying concete for sidewalks, taxiways, and runways have a cement truck show up, and for paving after it's been laid maybe a steamroller. Bigger structures like fuel depot and hangars could have loaders or other vehicles, and of course bulldozers for many things.
This is a VERY minor suggestion, and should be very low on the list if deemed a neat addition. I know this isn't a construction sim, and maybe these only show up for expansions which perhaps could also take a lot longer to build. Say construction is fast before you open your airport for the first time, then any major adds take a lot longer and are also more expensive. Could add a layer of challenge as well.