A couple fundamental things would need improvement.
1870s or 1880s style battles would need to be fought at extreme close range, so close that the exaggerated size of the ship icons could start to be a problem. Even at very low speeds, you would need to click a lot to keep the ships in engagement range. Close in battles might be much more of a mêlée than anything else. Formations as we know them might be fairly useless during the shooting part of the battle unless they are tightened up a lot so that all ships are actually in range of a target.
Ramming and boarding would need to be more possible/intentional. While ramming is possible now, it's somewhat random. Historically, ramming or boarding ironclads was often necessary to stop the ship (e.g., Huascar, Battle of Lissa). Ships were fully designed around ramming (e.g., HMS Conqueror). The solid shot and simple explosive shells of the time were often just not enough to actually kill an ironclad, and this is exacerbated by the low rate of fire and terrible accuracy.
Some nations would have very few ships or need to undergo some border and colony changes depending on when you place the start date. 1880 would be pretty OK, similar enough to the 1890 start. 1870 would be challenging due to the Franco-Prussian war that year. 1871 or 1872 could be the better bet, once Germany and Italy exist in their modern form. A lot of nations would still be in the process of acquiring colonies - as this is now pretty random in game, some means of targeting new colonial acquisitions may need to be implemented.
Ship classes would be quite different. Existing restrictions on ship classes would need to be thrown out the window. For example, the 1872 German Imperial Navy planned for "eight armoured frigates, six armoured corvettes, twenty light corvettes, seven monitors, ... six avisos, eighteen gunboats and twenty-eight torpedo boats." Should CA = frigate, CL = corvette, and B = monitor? What should be done to represent non ocean going torpedo boats, if anything?
There are various other smaller issues, more easily solvable but which also need considering:
- How will legacy wooden ships be represented?
- Will there be a separation of screw driven and paddlewheel driven ships?
- 18 inch muzzle loading guns in 1882?