I suggest following two new Castle Age Infantry units.
They will both share the same slot so every civilisations will only have one of them.
Anti Cavalry Shock Trooper
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Cost: 40 Food 40 Gold
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Training Time: 30 seconds
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HP: 60
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Attack: 8
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Attack Bonus: +8 vs Cavalry, +8 vs War Elephant, +6 vs Camel
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Rate or Fire: 2
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Armour: 1/1
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Armour Class: Infantry, Eagle Warrior
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Speed: 1.1
Anti Archer Shock Trooper
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Cost: 40 Food 40 Gold
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Training Time: 30 seconds
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HP: 60
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Attack: 8
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Attack Bonus: +4 vs Archer
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Rate or Fire: 2
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Armour: 0/2
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Armour Class: Infantry, Eagle Warrior
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Speed: 1.1
As you can see the stats are very similar. Cost, HP, base attack, armour class and speed are identical.
Only bonus damage and armour are different.
The total resources is the same as a Long Swordsman and 10 more then a Eagle Warrior or 5 more then a Ghulam.
The base stats are similar to a Long Swordsmen just the attack is 1 lower and the speed is 0.2 faster.
The anti archer version is basically a worse Ghulam, for 5 more resources you get 5 more HP but 1 less attack against archers and 1 less pierce armour, 0.05 less speed and on top of that worse Armour Classes.
But the 1 less attack doesn’t mater too much since both can kill Crossbows and Elite Skirmisher in 3 hits and both take 1 damage from Elite Skirmishers.
Also misses out on the extra tile of attack.
The anti Cavalry version is kinda a worse Kamayuk. 10 less HP, significantly less damage against War Elephants, but 1 more base damage, 1 more pierce armour and 0.1 more speed.
But the biggest disadvantages are the 1 less range and the Eagle Warrior armour class.
Both of the units have the shared trade that they are decent at raids since they are faster then most Infantry and Archers.
They are slower then Eagle Warriors though and also cost 10 more total resources.
That makes Long Swordsmen a good counter against them. Also most unique Infantry should be pretty good against them.
The issue with Crossbow + Knight is that Crossbows hard counter Pikeman and Knights hard counter Skirmishers.
But those two new units would each counter Crossbows or Knights without being hard countered by the other one.
But since they are mutually exclusive to each other (and to Eagle Warriors) they can’t be used in a potentially OP combination.
And they can’t become too OP since every civilisation in the game has access to Long Swordsmen and only one civilisation (Tatars) is leaking a Castle Age Infantry blacksmith upgrade.
Balance options
Anti Cavalry Shock Trooper
To make it easier to counter this unit it could get the Spearman armour class. Maybe with some resistance to it like +2 so Archers and Skirmiehrs would only make +1 damage but Calvary Archer with Parthian Tactics would make +2 damage.
Also removing the Pierce armour would be an option.
But I don’t want the unit to be too weak against Crossbows or else it wouldn’t ever be better then a Pikeman.
Anti Archer Shock Trooper
If it had 1 less attack against Archers it would require 4 instead of 3 hits to kill a Crossbow or Skirmisher.
Both
Maybe their weakness against Infantry is not strong enough. Giving them -1 or -2 Eagle Warrior Amour would give every Infantry Units more damage against them. (Some unique infantry would need to get 0 attack against Eagle Warriors for that).
Imperial Age
Not sure if those units need Imperial Age upgrades. They are designed to counter the Crossbow+Knight power spike in Castle Age.
Anti Cavalry Shock Trooper
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Cost: 40 Food 40 Gold
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Training Time: 30 seconds
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HP: 65
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Attack: 10
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Attack Bonus: +10 vs Cavalry, +10 vs War Elephant, +8 vs Camel
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Rate or Fire: 2
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Armour: 1/1
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Armour Class: Infantry, Eagle Warrior
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Speed: 1.1
Anti Archer Shock Trooper
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Cost: 40 Food 40 Gold
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Training Time: 30 seconds
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HP: 65
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Attack: 10
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Attack Bonus: +5 vs Archer
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Rate or Fire: 2
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Armour: 0/2
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Armour Class: Infantry, Eagle Warrior
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Speed: 1.1
Those two Elite versions would give them +5 HP and +2 Attack with a little more bonus damage.
The Two Handed Swordsmen gets +3 attack and +2 Bonus attack against them while the Champion adds another +10 HP and +1 Attack.
Unless they get some civilisation bonus they are now falling behind Arbalester and Cavalier.
Unlike Eagle Warrior or Ghulam they also don’t get faster.
If they underperfrom in Imperial Age they maybe should get 1.2 speed for the Elite version.
Civilisation specific considerations
Aztecs, Maya and Inca
Have the Eagle Warrior instead.
Civilisation with Infantry bonuses
Japanese
The 33% faster attack would make those units quit strong.
Considering that Japanese Win Rates are currently not that hight that might be a nice bonus for them. They struggle against both Crossbow and Knight civilisations so not sure which Shock Trooper would be more useful for them.
Goths
They have the Huskarl so they don’t need those units.
They would passively benefit from more civilisations using units that are countered by the Milita line.
Vikings
Their Infantry gets +5 attack against Cavalry which removes the need of the Anti Cavalry Shock Trooper.
Giving them the Anti Archer Shock Trooper might make them too strong considering their massive early Castle Age economic bonus.
Celts
Their Infantry plus Siege handles both Cavalry and Archers quit well not sure if they need those units.
Malians
The +2/+3 Pierce Armour could make those units pretty strong and they already have strong cavalry and camels so they also don’t need them.
Burmese
Their +2/+3 for Infantry would make those units too strong potentially and they also don’t really need a buff.
Teutones
Carried by a good selection of units and good economic bonuses they don’t need +1/+2 armoured Shock Troopers.
Slaves
Slavic Infantry becomes quit powerful in the late game and they have pretty good cavalry so they probably don’t need those Shock Troopers.
Bottom Tier Civilisations
Malay
They struggle on land maps as a Water+Infantry civilisation.
They have good archers so also good Skirmishers so the Anti Cavalry Shock Trooper could probably help them more.
They could also benefit from the Forces Levy unique technology turning the cost into 60 Food 20 Gold for a more reliable late game unit.
This way they had something to raid in early Castle and a Cavalry standin in the Lategame.
Koreans
War Wagons take a lot of damage from Skirmishers without them having any good cavalry to counter that threat.
The Anti Archer Shock Trooper would help them a lot against Skirmishers while giving them an early Castle Age raiding unit.
Vietnamese
They are basically the Anti Archer Archer civilisation so the Anti Cavalry Shock Trooper would help them against their Cavalry weakness.
Bengalis
Their Elephant Archers and their unique unit the Ratha is weak against Skirmishers, even in melee mode.
They would also benefit from the Anti Archer Shock Trooper giving them a hard counter against Skirmishers.
Dravidians
Giving them the Anti Archer Shock Trooper would make up for their leak of good cavalry.
With the Wootz Steel Unique Technology they would become quit good late game raiders.
Tatars
They have the weakest Infantry in the game so they could not really use either of those units effectively.
Italians
They need a Castle to get a good anti Cavalry option so they would benefit from the Anti Cavalry Shock Trooper. They could also benefit from the Anti Archer Shock Trooper if they want to use that in combination with their Unique Unit.
With the Condottiero they already have a fast Infantry unit so those tow units could combine well.
Summary
Most of the civilisations that could benefit from this unit happen to be in South/Southeast/East Asia. So maybe those units could become Asian regional units.
Giving this units to a bunch of civilisations would be a passive buff for Long Swordsmen because they now have a new role as anti Shock Trooper units and therefor it would be a passive buff to all Infantry civilisations in Team Games as well.
I know that this would be a big change but I’m still putting my ideas out here.