I'd had the same idea about Minutemen as a new UU for America... except I figured that they would best replace the Rifleman. (Given that the American Revolution was closer to the Riflemen era, even though they technically used muskets -- which IMO are mainly a late-Middle Ages unit.)
I think that the sea mines and minesweepers are a serious oversight in the game as it is; the manual mentions that blockades of enemy ports is possible; but it's actually kinda unrealistic -- at least in the way I play. You need to literally block every possible ocean square surrounding the enemy's sea borders with a ship... a ship that's got a chance of holding the blockade, too. Mines would be a whole lot more cost-effective, unpredictable, and just plain fun.
The cruiser idea is interesting, but I don't think it's really that necessary to have too many kinds of offensive ships in the same era. The only exception would be a
defensive naval unit, for patrolling your borders. (Once I get to Robotics in a long-running game, I usually try to cover my coastline (near the capital) with AEGIS cruisers to hold off the nuclear subs.
On a similar vein, what about giving ALL ships the ability to see subs when Robotics is discovered?
Another of the big peeves I've had is the Cruise Missiles -- those things pack some nice bombardment power, but IMO their range is far too short. I'd like to see them either (1) given a four-square range or (2) allow them to be loaded onto Battleships -- say, two or four of them. As it is, about all I do with them is stockpile several dozen if I'm playing from an island or subcontinent somewhere, to lob at any approaching amphibious assault before they reach the shore. (That worked to my advantage once; three transports full of troops were coming and their escort had sunk all my ships that could get there in time, but I managed to wipe ALL of them out before they even landed thanks to the cruise missiles!)
A super-battleship sounds like an interesting idea, but I'm not sure if that would work well in the game, historically speaking. In the real world, the battleship has played second fiddle ever since the end of WWII. I think they already have too much importance in the late-Modern Era as it is.