While I think the idea of territory control generating mana is a nice one. I don't beleive it fits in a game who's history states:
"the powers gathered up nearly all the magic of the world and concentrated them into a series of crystals that represented the basic elements of magic: Earth, Air, Fire, Water and Life.
In the climactic battle that resulted in the cataclysm, the crystals were shattered thus creating the shards of magic in which most of the magic of the world is now stored in."
This is why you need to control the shards (including life/death) in order to gain specific mana to cast spells.