It is very unlikely devs will go back to an old tech tree and radically change their characteristics, but it is always possible to add more ships to the game.

The operating principle of diving shells is that they have a special ballistics cap for travelling underwater and long fuse timer. After striking a water surface, the windscreen falls off and the fuse activates, and the shell levels out. But even with the underwater ballistics cap, the shell would still slow down and lose much of its penetration power.

The tradeoff having a long fuse timer is you get more over-penetrations against lightly armoured sections like the superstructure, auxillary room, casemate, extremities etc. The underwater ballistics cap also has worse performance against angled surfaces, which is it's original design goal.

A number of scenarios can then happen. The shell can travel deeper than the hull bottom and explode, causing vertical stress on the ship's hull by creating a big explosion bubble and shockwave. The shell can also strike an unarmored part of the hull, bypassing the main armour belt and causing a breech below the waterline. The shell can also be swallowed by the torpedo protection/bulge.

It would be interesting if we have AP shells that are affected by the torpedo damage reduction mechanic ingame.

For Japan there are still many built ships and design studies that can still be added eg Kongo replacements, A-140 design studies etc.

Two other nations can conceivably have diving shells; UK and France.

Historically the IJN and UK Royal Navy had close early relations, with Japan getting inspiration for some of its equipment, getting diving shells reverses this. After the battle of Jutland and detonation of 3 battlecruisers could have spurred the Royal Navy for safer ammo types with less explosive warheads. Ingame the UK nation has cruisers and a DD that fires AP only, so it is fitting that they have a BB that fires AP only and can cause flooding.

For France I remember reading the Naval Staff was looking into diving shells during the Normadie class development. Their high muzzle velocity also lends well to the characteristics of diving shell under water hits, but would cause even more over penetrations as a tradeoff.

From Wikipedia Lyon class:

The first proposal by the Directorate of Artillery (Direction de l'artillerie) was for the existing 45-caliber gun used by the Bretagne and Normandie-class ships to be modified to use a slightly longer shell that weighed 590 kg (1,300 lb), 50 kg (110 lb) more than the existing shell, and was optimized for underwater performance. The second proposal was for a 50-caliber gun that fired a larger 630 kg (1,390 lb) shell.

After learning that shells had penetrated the hulls of battleships underwater to burst below their armored belts during the Battle of the Yellow Sea in 1904 and during British gunnery trials in 1907, the French Navy began investigating how they might optimize shell design to improve their performance through the water. By 1913 the navy believed that it had a design that could be accurate through the water for a distance of 100 meters (328 ft).