I think there should be a battle of the Atlantic Game mode. Where 4 Destroyers will have to protect 9-12 supply Ships and there will be 4-5 subs ( maybe less). To win the Destroyers will have to bring 1/2 of the supply ships to the other side of the map. For the submarines to win the have to kill more than 1/2 of the ships (Destroyers don't count), If the subs kill more than 1/2 of the fleet the game does not end they can keep sinking more ships until they get killed or the remaining ships make it to the other side of the map. The subs will spawn ahead of the convoy, and they will get 2 chances to attack the convoy.
I am thinking that the convoy will be quick so that the subs will need to surface to keep up but if they do they will get shot at by the supply ships and Destroyers. The convoy will go around a very large island and the small subs will take a short cut through little islands and end up in front of the convoy and deal 1 more major strike. The subs can still follow the convoy but they will need to surface. I am thinking it will be with tier IV ships because the first subs come in at tier IV. Perhaps every supply ship you sink or successfully sink or protect you have a chance to win coal,steel,and oil. This is just a suggestion and nothing is set in stone all the numbers and maps and all the details are subject to change. If you like the idea or have any suggestions for the game mode leave a comment,Thank You!