Subs sound cool in theory but after playing the Silent Hunter series there is really no way that they would be fun in this game. Without time compression, which obviously wouldn't be allowed in Aces High, you would spend many many hours in the sub before you could even get a shot off on a ship. I'd really love to see subs, but I don't know how they could be implemented in a fun way. I guess they could be AI controlled like the CVs and a player can take control of them whenever they feel, or maybe make them cost a TON of perks and let them spawn from CVs and naval bases. Would be cool, but I don't think it will happen.