Been thinking on this a bit more, and came up with 2 similar ideas.
1. This idea works on the fact that most of the imbalances in equipment are caused by armor, rather than the weapon (That is, generally having a higher end weapon gives you less of an advantage than higher end armor). So, add a "Heavy Armor" skill that requires 3 str per point. It would work as an additional requirement to just str to wear the item. For example, Gothic Plate might take 15 strength (as it does now) and additionally 5 or so points of the Heavy Armor skill (I realize that the strength wouldn't be necessary in this case, but in the case of a lower tier, but still high end armor, such as Coat of Plates, which takes 13 strength, might only take 2 points of the Heavy Armor skill.)
This would essentially reduce the number of skill points player who want to wear heavy armors have, and does so in a fairly realistic way. Due to this, they would have to choose between having less health, speed, damage, etc. It would also have no real effect on Archers, who really don't need any nerfs like this, and while it would effect heavy cav, it would do so in a less drastic way than making them put in enough points to afford their already very costly horse.
2. This idea works on the fact that even with enough protection, you're still a sitting duck if you can't move at more than a crawl. Basically, increase how much weight reduces wpf and movement speed pretty drastically above a certain point using a logarithmically scaled multiplier, starting at weight 10-15, and nearing it's peak at around 30-40 or so. So, for example someone with a weight under 10-15 or so would see no change, but someone around 30-40 weight (heavy armor all around) wouldn't be able to move at more than a walking pace, and would have a HUGE wpf penalty.
Now before you stab me for trying to make heavy armor worthless, this effect could be counteracted by using a skill. This skill would require 3 str per point, and would reduce this extra multiplier by a certain amount (If the skill is high enough that it reduces the multiplier to the point that the character would be less encumbered than before, the skill caps out and stops helping more) So, like idea 1, if a character wants to use heavier armor and not be basically useless they need to put points into this skill; and, once again, they would need to sacrifice damage, agility, health, etc, in order to do so.
This idea doesn't really do anything about cav, and might be harder to implement, but I still posted it because I'm not sure it's possible to have 2 requirements on armor (i.e strength and Heavy Armor skill).
