1 mana per turn every three levels of beast sounds extremely excessive to me. But I think one mana per turn per beast sounds excessive.
Aye, it sounds excessive. Let's put it this way. I wouldn't mind paying 4 mana for a lvl 12 Great Wolf. It is very useful. Same with Corpse Spider, Umberdroth etc. Would be a nice limitation for the number of the beasts on our side.
I did not go into details, but you think right, summoned elementals/beasts have a limitation on their number, so I would not change the mana req for them. Plus, they are "created" by us. Exactly.
About using mana to "feed" beasts: I did not give details here either, but I do not see it as a real problem. A summoned Shadow Warg upkeep is 1 mana. It is an animal, summoned by a spell, so keeping it under control is 1 mana. However, a beast of the wild is part of the world we try to conquer, born in wild, keeping it under control should be more difficult.
Hero vs. beast: heroes fight for you for their own good. Fame, adventure, whatever. 1 gold upkeep is really low ofc for a lvl 20 hero, but they are heroes. Special kind. 7 mana for a lvl 21 Great Wolf (probably around lvl 15 if the sov is 21 so it is 5) howling for +21 attack for the whole army is just nothing in my opininon.
Anyway, we are just discussing ideas, this is mine... I am sure it will not be implemented in any way (with the leader/beast level % thing), I was just describing how I would solve the BL+beasts challenge.