I think it would be better if it was more freeform. I do not like the
forced archer/infantry/cavalry restriction. The tier idea fits though and IMO can be implemented without forcing restrictions on people. Let me try and make a simple example.
So you got a class this is the origional basic class as set out by Arch3r (now defined as a Tier 2 with infantry/ranged/cav limitations).
Could take any equipment in any combination and was allowed 50 skill points. This would be one step up from militia who may or may not have equipment limitations due to lower rank.
Essentially this Tier 1 "Soldier" is the potential jack of all trades you could make him into anything. He is the base for all further troop classes.
Maybe this would be unlocked by a barracks (I cannot be bothered to look over the buildings you have specified Arch3r so if you have an equivalent assume I mean that, nor is it important what the building is to get the point across).
You start with Militia.
So a barracks unlocks you a Tier 2 soldier class base. I already explained how it can use and be anything.
Then when you build an armoury/fletchery/stables you unlocked the infantry/ranged/cavalry upgrades. Except now they are a little different, they along with whatever other upgrades we decide to add unlock new class types.
This would be:
Tier 2:Archer/Cavalry/Infantry
These guys are allowed for example 60 skill points. However limitations as described by your earlier Tiered examples of troops apply. eg. the infantry are not allowed ranged weapons or horses.
They may also get class specific bonuses eg. 2 points in powerstrike for infantry or <30(tier 3?) armour rating armour(provided you have the materials and other requisites yet to be determined).
Or maybe these bonuses will be conferred by further upgrades at the armoury as suggested by your armoury example from your post Arch3r, however I think it would be much simpler for everyone involved if rather than having small individual upgrades they were contained within more expensive and widely bearing upgrades. It would be much harder for people running the game to keep track/update and much more annoying for the clans having to continuously edit their troops/versions of the mod. It would be much easier to include them in larger upgrades.
Then maybe when you build a keep you get the tier 3 soldier. He has the same jack of all trades style but now 60 skill points. No penalties and no bonuses.
The keep also unlocks the tier 3:archer/cav/infantry upgrades, they would have to be bought from their respective buildings. They would also convey similar bonuses to the previous tiered specialised troops but maybe larger and be given maybe 70 skill points.
This system could very easily be expanded upon with keep -> castle castle -> citadel etc. upgrades. Allowing us to stretch the upgrade paths for troops ever further should we want to.
Also you could maybe even include hybrid classes for those that want some degree of specialisation but not fully.
Maybe if you have a keep, an armoury and a fletchery you can make the "skirmisher class" a 60 skill point tier 2 unit(as opposed to the tier 3 specialised units with 70 skill points the keep also unlocks) with some set of bonuses and advantages that fit the idea eg. +1powerstrike and +20 ranged weapon proficiency.
This would also be a good way to implement horse archers.
------
If you were worried everyone would be running around with tier 2 soldiers all turned into hybrids anyway you could make it so your only allowed 1 single tier 2/3 soldier class. Then you have to unlock more class slots through upgrades. If you want to get all 3 specialised troops (were having a max of 4 soldier types + officers right?) you must remove your tier 2 soldier class from your roster. Then of course if you went for a hybrid class or tier 3 whatever you would have to "overwrite" an old class as well.
Now I do not know if this would be a better system or not(they are both untested after all) but it would allow us much more freedom IMO. It would also reward specialisation rather than forcing it which is how I feel it should be.
Also any values or names or specifics I used are not important so do not argue over them it was just an example to get an idea across.
------
As for armour I think there should be a blacksmith building which uprades armour city wide for all classes produced there or kingdom wide if necessary. You should start with for example Tier 1 armour. Then you will be able to unlock Tier 2, 3 etc from there also(or maybe start at tier 3 and upgrade from there). Maybe a good way to set the tiers is Tier 1 as <10 2 as <20 and so on. However this does not apply equally for each class.
It would have to work in 1 of two ways I think. It would either specified in the class base (eg tier 2 infantry, tier 3 armour. tier 2 archer, tier 2 armour) or alternatively paid for with the skill points used in troop stat generation. eg (tier 2 archer) tier 3 armour 10 skill points or (tier 2 infantry) tier 3 armour 5 skill points.
Oh and there should maybe be mines on the world map which apply +5 +10 etc armour quality depending on the material.
------
Anyway I think I might have failed at "simple" this ended up quite long although it is not that complex there is just a lot of it.
Edit:
I like paint!