Hate to tell you, but large caliber powder guns are simply obsolete. For land attack, it is much better to have the capability to change firepower from very low to very high. This normally can be accomplish with a variable rate of fire. Extreme accuracy is essential in all cases. There is a need for penetration too, but this most likely would be best accomplished through a change in velocity, not shell weight. A 5inch accellerated into orbit and falling down will penetrate, but it can also be launched at a flatter trajectory if penetration is not required with less velocity, and fired more often if broad area coverage is required.
I am more open to the idea of armor making a comback in surface ship design. Highly damage resistant warships have a broad appeal, think of the MRAP in Iraq. Protect the crew, prevent mission kill, or worse, total loss of the ship. It may become increasingly difficult to prevent a weapon from hitting the ship (some of the new missiles are scary, torpedos and mines are already really scary), so maybe we should look to how we can take the hit and move on? Missile (and perhaps direct fire weapons like rail guns) kinetic energy will increase, but I believe only at the expense of accuracy, making the weapon designer choose between vulnerability to countermeasures and penetration. In either case, more armor (and residual watertight integrity) is alway better for the crew
My battleship would have LOTs of electrical power generation capabilty (nuclear power would probably be required, plus they are closed systems requireing fewer armor penetration points), numerous small caliber, VERY high velocity (but variable velocity) guns capable of high rates of fire, preferably rail guns. A combination of rail guns and conventional rocket firing guns (like the 155mm AGS) if GPS accuracy cannot be achieve for the rail guns (I don't know how the electronics could survive the launch). Low freeboard, broad beam, relatively shallow draft, four props like Queen Mary 2 (no rudder), electric drive, LOTS of watertight integrity (automated when possible, with counterflooding capability). An active torpedo defense if possible, with laser point defense, capable of blinding missile sensors (or pilots). Ah, one can dream, I am sure my design would only cost half a trillion....
