Thx for all the reply's!
There already is a building that makes all enchantments on the city free, but for some reason this bonus is hidden from the player. Probably needs to be added to the tooltip.
I like that but also did not notice. All kinds of "all free" are difficult to balance, what about first two or worth of 4 mana.
I don't see the point of giving out 5 free mana upkeep. That is the same as 5 mana per turn, but needlessly complicates things. Better to just give mana.
It is not the same becourse it is bound to a hero and purpose. This could be good for warrior chars, or mages which need spell resistance.. Opposing to free mana for the pool this mana is less flexible, therefor you could give more of it for each perk. To remove complication there should be a net sum of what all enchantments on a hero cost.
The sovereign being the source of all magic, would have no limits, which is good for answering the question "What is so special about the sovereign?"
I agree a soverign should have a significant higher cap, so he will bring all the big pain. But again no limit here is a problem.
My whole point is that having a mana bank and no limit to mana spending I could cast 100 fire elementals in one turn. I do not think this is a good mechanic, spamming again. Also it requires more planing which is a lot of fun.
Not using mana is a bad choice.
Agreed. And i want to punish it even more 
About the global mana cap. I feel mana should be more like a stream than a pile. I do not want to remove all tactics using a lot of mana or spending a lot of it fast. But this should be something i have to plan, pay and work for. I require buildings for a big mana bank and also a "ritual pit" (or some thing like that) to cast it fast (or a global enchantment, or item...). In this way it could be easy to summon a lot of elementals near my home town but not at the front.
For gold it is partially the same. I can not buy 1 Mio. tons of grain or 5000 swords right now, even if i had all the money. It is simply not available. There are not enough smiths to create it instantly. For buildings and units it is this way, ok you can rush them but this is realy expensive.
May be limiting the spendable mana per combat/strategic turn is enough to disable misuse.
Some nice ideas, but seemingly crippling especially to alot of tactics, If I am to use midnight stones or spider webs for spells they should at least be for sale in the cities.
This would in fact only add a cost in gold to spells, but more planing i guess. I feel such consumables should only be used for very mighty spells, or even spells you only cast one to two times a game. Or one could have a soul catcher sword which provides souls of the dead to cast ....*getting lost again*
Greets!