It would be really neat to see some more types of small ships like frigates, Corvettes, large patrol vessels, and torpedo boats/pt boats. This would give smaller Nations a way to build larger navies and these boats could have reduced manpower costs as opposed to destroyers as well as increased speed. 
 There are also a few technologies for these vassals that could be added. For the PT boats, you could add a special research module allowing you to make aluminum PT boats if your country doesn't have that much steel. Another cool feature would be various types of light batteries such as 5-in, guns and 6-in guns and being able to choose between a single battery or a dual battery. Lastly, having the ability to mount early anti-ship missiles onto these vessels would be pretty neat as well turning them into missile boats or fast attack boats.

I know the mosquito fleet mod does exist but it's not compatible with mods like extended tech tree or rt56