To start off the idea, ill section it off into what could be reasonable or not in it. The basic summary of the idea is basically cargo/passenger planes and floating factories. Although we do have trains and other vehicles, I think planes would be a great addition not because it would be faster, but as the game progresses, you could have multiple airports around the huge open world map to deliver supplies to a train station base, regular factory, etc. Another part would be the floating factory idea. Although this may seem absurd, and perhaps very difficult to execute, it would be cool to have areas where planes can land, drop off ores or other items, have another plane area to construct the finished products and then fly to the final destination. No different than a ground factory I agree, but its just more for the cool idea for more possibilities. Now to go more in detail with each one plus small things.
1. Planes will be fuel heavy so to give it a better chance of being in the game, so other vehicles wont be useless, heavy fuel would be needed OR perhaps a fuel that only planes use that is either made from regular fuel or a different type of natural resource. (Idk what jet fuel irl uses, but if its not oil, maybe that?) Planes will also be able to carry up to 40 items for the cargo variant and for the passenger variant up to 10. Passenger variant is fuel efficient, faster, and basically just for travel while cargo is for delivering items. More than likely cargo would need to be researched in different milestone. Like say putting jet fuel in the MAM will unlock a milestone for cargo and perhaps more stuff too.
The whole idea above will count for 1 or 2 milestones depending on how it is executed.
2. Airports are basically like a vehicle station but for planes. They land and take off from there, drop off item, and refuel. Nothing too fancy. May be the opening to the planes in general by eithering be with the passenger plane or a tier on its on that unlocks jet fuel too. It really depends on what works best.
3. Drones that use electricity. Drones have 3 items slots and are decently fast. They can be automated or manned. They are charged through drone charging stations and deliver items quickly. Could be used for fuel for refuelling other vehicles by sending them to those. Really depends. Just a small addition. Also charging stations have an input and output part for delivery. If 3 slots are too overpowered for something so small, 2 can work.
4. Flying factories are basically what they are said to be. Factories supported by antigravity platforms. Could be needed another whole tier for antigrav or MAM research for antigrav. Really depends on how the devs would want to execute it if they added it. Anyways, flying factories would be beneficial not only for aesthetic design, but also because you dont have to destroy the landscape as much as you already have to in area. The height of these would vary and to access them from the land would either be by plane or an elevator lift. I would also suggest that with these, hangar doors would be needed for the planes. Would be a cool addition. Flying factories would have different foundation and walls too to not only add for function but diviserty in design since there are only two variants.
That is pretty much all i have for now. It was just something i thought would be cool.