Zao is one of the ships that have been powercrept again and again.
Before I talk about other things, I will talk about Zao as it is.
Zao is a Tier-X heavy cruiser with trollish citadel armor, powerful HE shells, great accuracy and best stealth among heavy cruisers.
Most ships have a trollish citadel armor, of all kinds: Spaced Multi-layered armor, Underwater Citadels, Hard Angled Turtlebacks or even simple "thin" armor which causes AP to over-pen. Powerful HE shells? Goliath got same powerful HE shells which actually pens more than Zao's. In terms of accuracy, the rest of the ships can get away with regular accuracy due to the amount of fire power they can put out. If someone wants a stealth cruiser, just get Minotaur instead. Sure, Zao can sneak up to an another cruiser but it will get rekt as easily as a Destroyer. And Zao can't even win any kind of HE trade which is supposed to be the feature of the line and rather has to both-broadside AP trade and citadel the enemy way often than you get citadeled.
Zao just paid so much for her gimmicks. If someone says 'Zao is balanced, every other ship is not', that is just looking at constant numbers.
If you are saying something is "Balanced", you are comparing it to the current meta. The current meta has a lot of powerful ships, so in order for a ship to be balanced, she needs to be powerful.
Of course, "Powerful" and "Overpowered" are two different meanings. "Overpowered" means a ship as too powerful for her own needs. For example: If a ship as powerful as a Tier-X ship has been placed in a lower tier, it would be uselessly overpowered.
Let's look at a few examples for how powerful the ships of the current meta is and how powerful Zao needs to be.
The first examples are just in the lower tiers of IJN cruiser line itself.
Furutaka and Myoko are the ships I'm going to talk about. Everyone can agree they are very powerful. They have the same features that the rest of the line has such as: High Damage HE shells, Accurate Guns, IJN Torpedoes, Good Maneuverability and Outstanding Stealth. Above all that, they got unusually high hitpoints. If you argue 'Ships above Mogami gets low hitpoints because they get Concealment Systems Modification 1' then, Every other ship gets the same upgrade and they also have better hitpoints.
The next examples are Hindenburg and Goliath.
Hindenburg is basically the mirror-opposite of Zao. Strangely, Hindenburg gets away with better HE DPM than Zao does. Ofcourse Hindenburg lacks the stealth but she still got many gimmicks of Zao as well as her own. Goliath is an another HE spammer. The RN superheal easily makes up for Zao's stealth and speed etc which Zao uses for her tanking tools and still Goliath can tank better than Zao in all cases. And Goliath still have many extra gimmicks like 16mm overmatch and improved HE pen.
The next examples are the Light Cruisers.
Zao uses her stealth to sneak up to somebody's side and shoot some AP at the exposed broadsides, but Dev-Strikes in a Zao is not something very common. Even if she gets radared, Zao can shoot and run away unlike destroyers. But Light Cruisers have better stealth. Although they can't deal massive damage as easily as Zao can, they can finish off what they started and kill the enemy unlike Zao which has to run away and re-locate after one good salvo. Even as a Heavy Cruiser, which can easily kill a Light Cruisers in short or long ranges, Zao struggles to fight against even Light Cruisers in any ranges. And Light Cruisers still have better HP than Zao.
So you can see that Zao is clearly lacking. It has a similar amount of good and bad features but the rest of the ships has more good features than bad features.
But what to buff Zao for?
There is two ways to buff a ship: Buff anything or Buff where it is lacking.
Buffing anything is what WG have been doing to Zao for quite a while. It is making a ship to extremely specialized like Smolensk which is balanced by nature(same ratio of good and bad features and gets rekt easily on misplays which is why you see this low WR on smolensk) but frustrating to fight against or even play it.
Buffing where it is lacking will make Zao more versatile and more heavy cruiser-like. It makes a ship versatile like Des Moines(maybe not as versatile but you get the idea) which doesn't punish bad play but doesn't reward good play either. It also makes a ship a worthy opponent that anyone wants to fight against.
What I'm asking for is just buff the Zao's hitpoints. It certainly won't change the stats of Zao players that much(except the initial burst of good players returning to play this newly buffed ship) since people who like to HE spam from the back would just play safe would stay at high HP most of the match and dies quickly when the enemy comes to them. It just makes a better ship that can reliably fight in the front lines and combat this passive gameplay. Just Buff the HP by 5k, not a lot but it certainly makes Zao stronger than a light cruiser. But I feel that 9-10k might be optimal. Ibuki and 203mm Mogami also shares the same fate as Zao as well and they should propably be buffed accordingly(not by the same amounts of course like propably 203mm Mogami 1-2k and Ibuki 3-4k).