ok so for me it would go like this :
subs would spawn in between the coastal and DD spawn and the BB spawn, maybe a bit closer to the battlefield because of how slow they are.
Submarine players are then faced with a choice : transit on the surface, with the risk of being spotted by a DD or a BB, or transit underwater, making them slower and reducing their impact on battle.
meanwhile a DD can either fight other DDs, or do the usual PT boat hunt, OR it can chase submarines. if it choses to chase submarine it means it has to get closer to enemy BBs, which can makes the sub hunting more difficult. One thing it can do however is spam smoke, since it will use sonar to hunt the sub anyway. That will make it more difficult for bigger ships to have a firing solution on it. Note : CAS can also be a pretty potent threat to submarines, and if they have no ship in the vicinity to escort them, they can only dive and wait
Meanwhile BBs are also faced with a choice : either they can hunt the DDs who are hunting subs, but that means they are not focusing on other BBs and that might end up being a burden for their team since they don’t remove enemy guns from the battlefield.
Or they can focus on enemy BBs at the risk of letting their allied submarines die.
They can also try to hunt for subs at periscope depth, since i doubt a few meters of water will stop 300mm + shells.
i know all of what is written above is quite idealized, and it may not work that well in practice, but i think it’s worth a try. and it won’t make naval worse anyway, unless they decide to put the sub spawn right next to the enemy BB spawn, in which case, yes, subs would be OP. But let’s be fair, it’s very unlikely to happen
EDIT : i’ll try to post a generic map later when i have some time, so that my idea is clearer
As for aircraft carriers with entire AI or player controlled plane squadrons, i think they should not come.