Power and air condition units: will GSX ground crew be able to handle and connect GPU and the air condition unit on the plane itself? Are the cables visible? That would be an amazing feature for sure!
While P3D4 already has an API to create wires, they are a bit limited and too simplified in appearance so, I think it might not look very good to have a cable which looks textured and modeled, then it would become a simple wire, in order to use the new "Rope simulation" system which has been added fairly recently in the P3D4 API.
I guess we *might* create our own cable simulation, but it's not the easiest things to do, and it would still be a P3D4-only feature anyway, since it would require an higher number or bones, and we are already close to the maximum possible with FSX. So, maybe not in the first release, but after allowing more time to finally have everybody moving out from FSX.
Do you find some limitations here (for example only a limited amount of passengers can be shown at a time)?
When something doesn't look entirely realistic, always ask yourself "could be they made it like that for performance reasons ?", and you'll find the answer would be almost invariably YES. Yes, we need to keep the number of passengers visible at any given time under control, which is why we try not to form too crowded areas.
Don't forget the situation we started with: an empty bus and passengers created by your own imagination, which is GSX always worked so far.