Mana-Cost-Reduction effects from enchants (and guardian power buffs) now affect mana regen instead (so if you had a -20% mana cost enchant, it's now +20% mana regen)
My poor, poor 40-mana character
Not sure how I feel about this one. Shields already make Mana a really, really good stat to upgrade. And now it's even more important, since Mana Regen is % based. It's kinda turning into a "High Mana Characters only if you wanna survive AND deal damage at the same time" thing.
Characters with high HP and very low mana are almost worthless after this change. So Medieval and Skelebot characters are going to be VERY seldomly chosen.
To compensate, a new type of Head Enchant that boosts max mana might be nice. You could have it boost by a set amount instead of a %, so you could more-or-less control what spells it would allow low-mana characters to cast. This would make a nice feel of "Leveling Up", where once you get better enchants, you're able to cast bigger spells that take more mana.
"Tiering" the mana costs of spells at some point in the future might help too. Different spells would scale up in costs at set intervals instead of having seemingly random costs. Weak, rapid fire spells could be 25 mana, for example, while bigger projectiles would be 50, whips would be 75, AoE's and more powerful projectiles would be 100, ect. Some sort of set cost instead of "Light Orb costs 27.9 mana per cast" would really kinda tighten things up, IMO. But that's an optional idea for the future.