What I'd like to see at some point further down the line would be a bit more variation/options in the guns we have access to.
Right now, a 6" gun is a 6" gun, period, and even gun sizes only go up in increments of one full inch (there were obviously tons of guns historically that fell between these neat increments). On top of that we have shell weight, type of powder, and an arbitrary "advanced loading/autoloading" buff option.
There's a lot more to it, for example in barrel length (one of the best examples are probably the USN 5"/38 vs 5"/51, but also the various kinds of 16" guns from Colorado to Iowa), actual autoloaders like on the Des Moines or some post-war 6" cruisers that gave a lot more than 25% better reload, shells becoming a lot better over time, and others.
Not a high priority issue at all, but at some point I'd kinda like to be able to recreate more of the intricacies of historical ships 🙂