For this mode, I think it should have Triton style movement where you have to commit to jumps across leagues.
And this bothers me a bit as well. I'm pretty sure if creatues like Ents/Gryphons or even Zombie Bosses themselves enter Mana Pools, the Mana Pools only become Blighted and there isn't an additional hero spawned.
Although Magical zombie creatures would be awesome (... A mage Zombie Dragon...) I think what could be in place is that Mana Pools get a 3rd state if destroyed by a creature that doesn't spawn a hero. So if a Zombie Gryphon flys over a Mana Pool, it will enter this 3rd state that turns any zombie that can spawn a hero that walks over it again to turn into a zombie hero.
These 3rd state mana pools could also have a high piority to zombies in the immdeiate area I think. Like Zombie AI could look for closest settlement, then look in a 2-3 league area larger and check if there are any 3rd state Mana Pool. If there is a 3rd state mana pool in that second search, it will be drawn to it to spawn into a hero.
If a player reaches the Mana Pool before a zombie does, the Mana Pool could turn into a normal Blighted Mana Pool.
Quick MS Paint mock up of what this 3rd state could look like.
Purple water with a whirlpool effect on it