For the original post's idea - I definitely like the idea of multiple turrets, but it seems to me like there might be a lot of other stuff for the devs to focus on before they were to even think of it. Although, in the future it'd definitely be cool, and make replicas look better. I do think that some turret additions might be more necessary than others. Ex. nations without the right turrets at all for a certain era, or secondaries still having gunshields with 0 secondary armor (or destroyer and CL turrets having shields with 0 turret armor, for that matter)
For DougToss' casemate idea- I think this would actually be very easy to do. They could have armor plates to cover unused casemate slots... and then when a gun is placed behind it, it disappears. They'd only have to use the same code as they do for lifeboats (that disappear when a component is placed near them). This solution would be visual only, but it'd still be really cool, and more realistic compared to what we have now when we decide not to use all the slots, which is empty holes in the side of the hull.