Any changes shouldn't be around realism vs "arcade", whatever that means. It's a video game. If I wanted a war simulator I'd join the army or go with the Doctor to kill Hitler.
Changes should be around whether they add to, incorporate new, or enhance existing systems and strategic thinking required of the player. Doesn't mean it's exclusive to realism, only that it should be the analysis applied to prospective changes.
With that in mind, I think motorised supply using fuel is a good idea. It adds a new aspect to the decision making choices of the player (increased supply throughput vs the increased use of fuel). From my hazy memory, I believe slow, horse-driven supply lines were a problem from Germany, as they needed the fuel for tanks on the frontline (so there's a basis for this decision being given to the player).
Likewise, I think a pilot pool is a sensible addition. Pilot loss was a big issue during the battle of Britain, so there's a historical basis for adding this metric to the game. It would come with leaders that could increase pilot training rate, or otherwise you could add a sort of wing template (like division template) that needs to be trained with equipment before they can be used. (Side note, rushing the deployment of these wings has a basis in history from the battle of Britain, I believe some German pilots had as little as a few hours flight time before fighting).
Alternatively, maybe sitting a wing in a home territory and practising manoeuvres could sort of be the equivalent. In which case the experience modifier for air might need adjusting, or the training mission might need to have a higher cap for experience level.
I'm not aware of any other areas where you need a pool of experts. Anyone can drive a tank, load a cannon etc and I think that training is already reflected.
I personally would like equipment supply to work a little different. I believe it all magically appears in the capital before traversing the network. But this makes no sense. For example, if I surround an industrial city but don't take it, the enemy should be denied the equipment produced in there. Additionally, any enemy infantry stuck there, should get their pick of the equipment.
Happy to be corrected on my understanding of equipment supply though.
I've a similar issue with sinking convoys that are transporting troops. Some how sinking every convoy doesn't drown every soldier? I'm not sure how divisions in north Africa are being reinforced when I've denied the Mediterranean to the British, but maybe I'm not understanding something.
