"In order not to make the game lopsided you can make them only available on premium ships as a premium item"
Not make the game lopsided? Excuse me, but that sounds kind of pay-to-win... You have to have a premium ship for it? And it's a premium item?
That aside, more aircraft should have access to DC's, because plenty of aircraft had such in WW2. In fact aircraft had access to sonobuoys and other mobile sonar methods consequently having their own sonar. IMO depth charges should be made available on tier 2-4 scouts as well as a large scout.
So instead of making it a premium item, or a tier 6 scouts, you can create a "Large Aircraft" pilot line that breaks off from the Rookie pilot, preferably after the Recon Pilot. Let's say, level 55 or 60? And this "Large Aircraft" pilot would be able to control the transports of each nation (ie C-47, Ju-52), though maybe later down the line. Starting with the large scouts mentioned here, and moving on to the transports a little later and then a tier of medium bombers (B-25's for example. Doolittle raid!!). For laughs and giggles, why not put in a heavy bomber like the B-17 or B-24, the Lancaster, the Me-264, the G8N Renzan at level 100 or so, that takes up far more space than a T2 dive bomber (say, 300 space compared to the 120 a T2 DB takes), two or three deck spaces when being fueled instead of one to avoid spamming, and a longer fueling time too to make up for the decreased numbers that would be fueling. Also they carry multiple bombs; these bombs would be lighter than the single bomb carried by a dive bomber, but they can carry several of them, and the player can choose whether to drop all bombs at once or one at a time (based on whether gradfire, max velocity, or simultaneous fire is selected). Why not implement them? We already have transports that can fly off of carriers...
