As a former navy guy, I'd like to see more naval units. I realize that too many units is more cumbersome and more micromanagement, but I feel it spices up the modern age. You don't want to go too far as the game is more than just the modern age. But...
Civ4 had submarines but not modern ballistic submarines (they had attack submarines). I'd like to see both. They need to beef up submarines to make them more viable. I feel submarines in every single civ game have been largely useless. The exception being able to carry missiles such as tactical nukes. That's important. If it were civ4 I'd say give them the ability to choose their target in a stack. But since civ5 doesn't have stacks, I'm not sure. But I feel even the existence of a submarine in an area around an enemy city should reduce 1 trade route. Note: this should be a passive ability the submarine still should be able to fight regular units and block a trade route at the same time. Submarines should be able to withdraw from combat without taking any damage whatsoever (say 15% chance depending on the type of ship attacking it)
destroyers. The upgrade path seems to be fubar'd in BTS. Iirc stealth destroyers can't see subs. In fact, stealth destroyers seemed useless to me in BTS. Have these upgrade to modern destroyers or fast frigates. Basically this would be a cheaper version of missile cruisers. Have them be able to carry one cruise missile, see subs, have small chance to intercept aircraft.
modern transports similar to LHA's etc. These should have some air defense capability and not be completely vulnerable to air attacks. Don't give them as much defense as aegis cruisers of course.
mines and minesweeper ships. Mine seem largely abscent from both water and land in civ games. Poorer nations should be able to mine their harbors to damage ships. This could allow civs with less tech to wound the better ships enough to allow their lesser ships to finish off the wounded ships. Minesweepers of course would spot and disable mines.
battleships need major reworking. They are too powerful. These should have a vulnerability to air attack. I feel civ5 is screwing up by making these things be able to bombard 5 squares. I'm tired of battleships dominating civ games... Bombardment should be made largely useless after the advent of cruise missiles. The best way to fix that I suppose is to really buff up cruise missiles.
carriers: I mentioned this before. But carriers should be able to "see" at least 6 hexes away, preferrably 10 hexes away
without specifically having to do a recon mission. This would help carriers overcome the battleship problem (battleships hanging around after the year 2000 etc.) This would allow ample time for your planes to sink the battleships. Yes planes should be able to sink ships. I'd have 2 classes of carriers. Early carriers and supercarriers. Early ones would carry 2 air units, super carriers would carry 3.
Aegis/missile cruiser: High aircraft intercept chance, spot submarines carries 4 missiles. Can intercept any air mission conducted within 2 or 3 hexes of the cruiser (depending on map size).
There should be nuclear versions of every ship listed above. Higher shield cost in exchange for more movement (say 2 hexes). I doubt they want to make the game more Alpha Centauri-ish, but I'd like to see it. Nuclear reactor technology necessary of course (this tech should come after nuclear fission tech)
I'd l like to second the post above. Dreadnaughts to bridge the gap between mid/late 1800's tech and ww2 tech.
I won't go into future units too much. It is a historical game afterall
