Some units may be redundant.
1) Instead of having Sergeant Spearmen AND Armored Sergeants, just get rid of the Armored Seargeants, give Sergeant Spearmen a base armor value of 2, and give the Sergeants three armor upgrade levels.
This will inevitably make their stats the exact same as Armored Sergeants while still retaining their lightly armored status at the early game.
2) Do you really need an Armored Swordsman unit when adding England to the Dismounted Chivalric Knights list will do just fine? Same stats and usage, even same price.
3) Do you really need two different units of Levy Spearmen for England and the Mongols? Couldn't you do away with these units altogether and give them spear militia in their place? Likewise, Mongol Foot Archers and Dismounted Archers are closely redundant, and could simply do with a combining and averaging of their stats into one single foot archer unit.
4) Why not do away with units like 'mercenary_blank' and simply apply the mercenary_unit tag to the original unit? Instead of two slots for Mercenary_Arquebusiers and normal Arquebusiers, have just one with Arquebusiers, but add the slave faction to it and apply the mercenary_unit tag.
There are several cases of this.
5) Remove peasants altogether. Town Militia serve their purpose just as well in both form and function. That's at least 4 unit slots free right there.
6) Free some space by removing some mercs that are neither recruited if at all by players or rarely seen, like Flagellants (I think I might have recruited a grand total of 2 of these ever since M2TW came out).