- Increase maximum submarine sizes to 3,000 blocks instead of their current 2,000- I say this because in the second world war, some of the larger submarines like the American Gato and Balao class submarines easily displaced as much or more than the standard fleet destroyer.
- Give submarines 1-3 more 4-inch guns- I say this because submarines in the interwar and even ww2 period often had enough deck firepower to sometimes outgun frigates and small destroyers and I think adding 1-3 more 4-inch guns would make submarines a little less feeble in terms of their surface-combat capabilities.
- Give submarines access to normal torpedoes instead of just submarine torpedoes- submarines in both world wars often had many more than 6 torpedo tubes and it would be interesting if submarines could launch more of the weaker standard torpedo if the designer wanted (obviously, work this around the current submarine-torpedo firepower system so that there has to be a tradeoff)

These would go particularly well with my frigate suggestion, as all of these are pretty much just buffing submarines.