Actually, I'm kind of rethinking my stance on the creature spawning. I'd say, not all mana nodes should spawn creatures. Only those where a creature seems to logically fit, like lightning elementals around air nodes.
I kind of envision the natural power of them, attracting elemental beings.
However, I don't for instance, think that death mana should spawn creatures. But instead, I'd like to see a demonic skeleton spawned whenever a living unit dies near it. And I don't think things like law mana should spawn anything.
In general, I'm all for mana nodes being more involved in the world. Things you'd like to be near, rather than just mere resources. Or in the case of some mana types, things you'd really not want to be near. I'd like to see Entropy mana spread hell terrain, and cause -1 food in all nearby plots.
Actually, spreading hell terrain would be a bad idea, don't do that.
But the -1 food, and also, turning forests and jungles into Burnt Forest. Just for the awesome visual effect. Perhaps death mana should do similarly. It would add a nice bit of immersion to have such "dead places" in the world, where crops cannot grow, and trees wither.