I'd like to see better ASW air units. Currently you can combine sonar planes and torpedo bombers, but it's fairly unwieldy - it's hard to get the sonar planes to loiter, and patrols tend to end up with the sonar plane seeing a sub and moving on before a torp bomber gets near it. Torp bombers also don't seem to make attack runs on enemies they encounter while patrolling.
To fix this, I'd like to see a gunship with a short range sonar and a depth charge dropper. Without a torpedo it won't replace the torpedo bomber entirely, though it could feasibly obsolete the sonar planes. It could probably work as a fixed wing unit if necessary, I only suggest a gunship because IRL helicopters make effective ASW platforms.
With this done perhaps torpedo bombers could get a slight change? I'm thinking something along the lines of more powerful but unguided torpedoes - leave ASW to the new gunship but give torp bombers some real hitting power against ships. Currently they have a tendency to die in droves and yet not deliver all that much damage.