Enchantment caps, researching needing to be dampered down, cultural units becoming irrelevant...
Honestly, they really feed into each other, don't they?
Hear me out: First of all, research & enchantment caps. Instead of having a blanket enchantment cap that simply applies across the entire game, would it be that much of an issue to add in a new research, gained at let's say, Tier 2 & Tier 4 tomes, that increases the enchantment cap? This way, players will be given a choice. Do they want to focus on rushing through the tomes to get to the higher power enchantments, units and buildings first, or do they want to pick up an enchantment cap increase to try and empower their units up further? To take full advantage of all possible enchantments, you'll certinally need to pick up the enchantment cap increases at the end of the day, but the choice of when you pick it up to take advantage of it, won't just lower the power ceiling of units, but also generally slow down the pace of research and add a new strategic element when it comes to research.
The question of Race Tranformations vs Enchantments, or if lower-tier units have a higher cap/increase their cap quicker, are a bit outside of the discussion, but important to take into consideration.
Then, Cultural units vs Tome Units, and how that comes to Enchantments. Honestly, this could be easiest to implement. You know how cultural units have their cultural abilities, like industrial's bolstering or high's awakening? And how they have an enchantment that allows non-cultural units to have that enchantment too? Well, that would mean natively, they already have one more enchantment slot than tome units, thanks to not having any need or desire for that cultural enchantment. But say that isn't enough, and you think the cultural abilities aren't strong enough on their own to help the cultures stand on their two feet, and it just means the cultural enhancements won't be used? Well, just say, because the Cutural units are the race most tied to the ruler, either natives to the world in the case of Champions, or having followed their ruler into a new world in the case of the other rulers, they have a higher enchantment cap on top of the lack of need to pick the cultural enchantment, and I think it would help make Tome vs Cultural units more distinct with strategic pros and cons to either side.
I also very much support the idea of Cultural T4s, even if they can't be recruited normally like the Feudual Knight.