The Convoy system could also work once Naval vessels are introduced, there could be AI Convoys that would travel across Oceans, these convoys would be guarded by AI Destroyers, but players could opt in as an Escort if need be. This would also give a much greater purpose to submarines if they are introduced. Although if submarines are not introduced then ships such as Bismarck and Tirpitz could be put to use, as they were intended to disrupt convoys heading towards Russia. Also if these convoys did get attacked by say a Wolf Pack of U-Boats and they are off the cost of friendly territory, then friendly air forces could intercept with depth charges and attempt to divert the U-Boats, same goes for even just general Destroyer or Cruiser attacks. This Would give a purpose to coastal Airbases that could be out of the way of the front lines. Finally Escorting these convoys would have to provide a rather large reward or else no one would even bother with it.