My thoughts on this is that there's a unity in design of each type of unit in the game that must be stuck to. For example:
Mainline Track: Warrior/Swordsman/etc.
None of these units have special abilities unless they are UUs. All have the highest strength in their era among land units. All move 2 tiles per turn until MechInf.
Specialist Track: Spearman/Pikemen/Marine/Paratrooper
All of these units have special abilities. All of them are weaker than the Mainline unit of the same era. Most come earlier than the Mainline unit of the era (exception being Marine/Ptroop). All are briefly useful as substitutes for Mainline units, but eventually are eclipsed by other units (or upgrade to non-infantry, in the case of Pikes).
Some units have one major change in design, such as archers losing their low strength but also losing 1 range when they become Gattling Guns.
Therefore, the chariot archer replacements need to stick to a single philosophy with possibly one major change during the track. Also, remember that the tree already has Medieval examples. As such:
Chariot/War Chariot/Horse Archer (Require a resource unless UU; come online later than same-era archers; higher melee and ranged strength.)
Keshik/Camel Archer (Require a resource. Come ~same time as same-era archers. Higher melee but LOWER ranged strength.)
So you have some choices. I'd lean towards the medieval template. I.E.:
Chariot Archer Upgrade Line Traits:
1. Should always require a resource.
2. Should come at same tech tier or one tier later but in a different tech than same-period "archer."
3. Should have higher melee strength.
4. Should have lower ranged strength.
5. Should have a range of 2 at least until Industrial era.
6. Cannot make melee attacks.
7. Have a specific unit against which they are weak (spears early, e.g.).
With this in mind, these would be my suggestions:
Medieval Era
Mounted Bowmen of some sort
1. Require horses.
2. Should come at Physics or Steel (these are the least populated same-era techs).
3. Melee and ranged strength should be same as Keshik (15, 16).
4. Should have 4 moves like Camel Archer, but no movement after attacking (to keep Camel Archer and Keshik unique). Range of 2.
5. Weak against spears/pikes/lancers.
6. Could theoretically be dropped and have the renaissance-era horsearcher moved forward to the beginning of Ren era. Advantage of dropping would be for historical accuracy, horsearchers being rare in era.
Renaissance Era
Cuirassiers
1. Require horses.
2. Should come at Gunpowder or Metallurgy.
3. Slight upgrade from Medieval, since not much further tech-wise. (20/22 melee/range?)
4. 4 moves, no move after attacking. 2 range.
5. Weak against spears/pikes/lancers.
Industrial Era
Cuirassiers continued to be used in this era in Europe. Could just continue using them, or could split into Musket Cuirassiers and Rifle Cuirassiers. If splitting:
1. Should come at Rifling. (Requires Horses of course.)
2. Gattling Guns are when "archers" stop having low melee strength. Therefore, the paradigm of higher melee strength should be dropped. Suggested strengths: ~28/28
3. 4 moves. Should retain 2 range unless we alter strength paradigms. If we drop to 1 range, suggested strengths: ~32/28
5. Weak against spears/pikes/lancers.
Modern Era
This was an era of static warfare, so nothing fits very well here. I'd recommend not having one for this era, but if we did have one it should:
1. Require oil or possibly iron.
2. Come at Ballistics, Combustion, or Replaceable Parts.
3. Strength ~50/50.
4. 4 moves, 2 range.
5. There's no specialist unit in era it can be weak against, but in next era could be weak against anti-tank.
Atomic Era
Lots of options here, but I advocate Self-Propelled Guns, such as the StuG. The advantage of using the StuG or the like is that it's a light artillery piece, thus justifying its range, and it has no turret, thus justifying forcing it to stop moving after firing.
1. Require oil.
2. Arrive at Atomic Theory, Combined Arms, or Computers.
3. Strength ~75/75.
4. 4 moves, 2 range.
5. Weak against antitank.
You could also add an information Era version of the SPG, possibly based on Paladin. Would arrive at maybe Robotics with ~90/90 strength and requiring Aluminum.
