When it comes to countries, imo the USSR needs a rework most.
But actually, what I really want, is a reworked air warfare system. I would like to see a new organisation system akin to the naval fleet/task force system, giving the possibility to organize my air wings into larger air fleets (call them air forces, or luftflotten, whatever). For example "recreate" the US 8th Air Force in England, composed of both Strat bomber wings and fighter wings, with active areas assigned where they will conduct start bombing and the fighters srving as escort, quite like the naval task force system. Also of course I want Air Force Generals to lead these Air Fleets, just like the Admirals and (Army) Generals already present.
As kind of cherry on top this could come with proper jet aircraft icons (instead of the black silhouette placeholders we have now). Actually, those black placeholders are the reason I really expected an air rework "coming soon" from the very start of the game. Well, I was proved wrong...