I agree very much with this, especially the idea of having to choose between enhancing units vs. casting spells. Anything that increases the number of viable playing strategies is great for me!
Of course, we might end up in a situation where the point is moot due to such an amassed amount of mana coming in that all spells can be cast and enchanted armor be created without any issue.
It might be a good idea to implement a mana cap to fix this, possibly upgradable through research.
