First I don’t think Skirms are really “OP”.
But if the meta is dominated by ranged units (or it’s necessary to make ranged units to damage the enemy) games will just naturally bottleneck in Skirms.
The reasons for that are various. There is no one single reason, but the most impactfuls are that Skirms are the “same unit counter”. They are the counter unit to all range units and are themselves a range unit. And that’s quite easy to add specific other units to Skirms to deal with everything they can’t deal with - and thanks to the ranged nature it’s hard for the opponent to clear all the Skirms before you can make the transition (especially if you put them behind walls) . And even if they do you don’t even lose a valueable army.
In low numbers Skirms can only herass eco, but in higher numbers with the range upgrades they even can start to kill vills. Which should possibly be addressed at some point aswell. Because with range and pierce armor if your skirm numbers are high enough you can position them on very abnoxious spots in the enemy eco - they don’t need to actively kill villagers, especially as a trash unit. Ideas could be that Skirms wouldn’t get an attack boost from the range upgrades anymore, instead they would increase their bonus damage.
Also the current meta incorporates walling (though it is not as much as it was some time ago), which also helps Skirm openers actually on both sides of the Walls. Which should also be addressed at some point. Walls could be more expensive but more durable in compensation which would make it more situational and map depending - also actively helping the more aggressive player cause they would have a bigger ressource lead to help their attack.
I also want to share to you a video of sitaux where he speaks about skirms, mentioning the issues above an more. He says he doesn’t think there is a “simple” solution. And he might be right, but I personally think that the addition of a Shieldman, when done right, could already solve the biggest issue - the strategical bottleneck.
So, why and would the Shieldman (which makes sense to be added as a (northern/central) european regional unit) solve the issue?
Ofc first the design has to fit. It has to counter Skirms. I personally think it would be ideal if it was resistant to most ranged attacks (including hand cannoneers to some degree). Ans have a bonus damage) against them aswell (possible to have it partially or only against skirmishers). If the amount of armor and health is well chosen (I tend to lean towards 2 armor) they should be extremely resistant to skirmisher darts and therefore really hard to kill for them. As the Shieldmen should be quite cheap, even if they are then killed by 1-damage darts you wouldn’t lose too much. And if you can pull them back, you can heal them up in buildings like TCs - as they are quite spammable you could do that more than with eg MAA, Scouts or Knights which you often only can make a few of because your eco is already pushed to the limit. (one point that sitaux also pointed out, that in many cases these days you just can’t get out enough of the counter units to clean the skirms - or even worse to even close the gap).
As the Shieldman is an Infantry unit in the early and midgame there is also no common unit dealing bonus damage against it. Though it would be the option to give it also the “shock infantry” armor class, so the swordsmen would counter them. This wouldn’t be an issue as the swordsman line is still the worst line in late feudal/castle age, so forcing the enemy into these can actually be benefitial.
In that constellation the Shieldmen could serve as a “bottleneck breaker”, allowing other units to be chosen to deal with them. Like Knights/Swordsmen, but even things like xbows/CA and even Siege could be viable transitions when your Skirms are countered by Shieldmen.
Ofc Shieldmen have no range, which leads to the unfortunate reality that they need to be able to tank absurd amounds of javelines to be feasible. And be cheap. Wich means they will need to have an absurdly low dps (against everything that isn’t a ranged unit) to compensate for that bulkyness. But that’s also something you should expect from a unit that only is equipped with a Shield.
There could be even the option to give Shieldmen negative melee armor as compensaiotn for the (for the price) high health pool required to tank that amount of javelines needed. They could get a small amount of Bonus damage against buildings/walls so you can pressure the opponent hiding behind them even without Arson already. But ofc this possible needs a lot of testing/tweaking because putting them against the walls makes them exposed to enemy ranged fire - a game of high attention where it would be crucial to draw back injured ones to heal them up. This interaction s hardly predictable in outcome before the release, as it is so heavily depending on actual in-game execution.
I only want the Shieldmen to be available to civs that actually historically used them. These are most north/centra european civs. But tbh i don’t know if there might have been some in Asia aswell. Haven’t heard about it but that doesn’t mean nobody had it. 2 Reasons: A) To solve the Skirm issue it’s actually NOT necessary every civ has access to that kind of specific counter, it’s enough if like 1/3 of all civs have it. B) I don’t want the Shieldmen to become a major line seen in every game and the game evolving around it being used. It’s a unit that should come into play when the meta evolves too much into Skirms as it does right now and breaks the strategic bottleneck of Skirm meta. It’s actually helpful if the unit is only available to a part of the civs so it is a requirement for their design to be not too useful generally. And it would be ideal if it was given to civs who themselves have comparably low skirms (which are for some reason a lot of these northern/central european civs).
One concept for shieldmen I made some time ago on a different topic you can find here under the “unique units” dropdown:
Nowadays I would possibly give it the “Shock Infantry” armor class, increase the Movement Speed to .96, add another +1 Bonus against Skirmishers and Gunpowder and give it -2 Melee Armor. There could also be the option to slightly reduce the HP (by 5-10) but give it some Infantry armor if it turns out Hand Cannons shut it down too much in the lategame in relation to other ranged units. (also it should stack twice as dense as eg swordsmen reflecting the low pop consumption and the nature of the dense “shield wall” formation)
But generally I think that’s possibly the way to go if we want such a unit actually working. We’ve seen too much that units who don’t have enough HP for their cost just get slowly killed by Skirmishers before they can engage. And we need to make sure this doesn’t happen for shieldmen too much (as long as you pay a bit of attention and pull back the injured ones). It’s a very atypical unit design with these stats, but tbh I don’t really see another way.
The reality is also that if we don’t make it half pop the HP need to be so high it would basically never be targeted by ranged units, which would defy the intended effect that you actually want be able to use the Shieldman as actual shield for other units that are more vulerable to ranged fire. But I also like it that way cause with the cheap cost and therefore only “average” HP for a melee unit, you can actually help it out with taking a bit care of it. If it had like 100 HP you would just patrol it in and hope it does it’s job cause you would get more value paying attention on other stuff. So I think the odd unit design actually makes a lot of sense in practical gameplay.
The shieldman wouldn’t solve every single issue with Skirms. But it could solve the strategic bottleneck issue that can be very annoying in a rts metagame like AOE2 is. And solving that would then allow to finally get into the small detailed interactions with Skirms that can/should be fixed at some point aswell.