Seems it's only happened once so far, Alpha Authority, though there's also Aspect of Mongoose. Both of those were untargetability + some other bonus for , which this is as well, so your costing seems spot on here. (That said, I'm sure this would be perfectly printable at if you wanted to.)
It was originally going to be you got a copy of the enchanted creature, but then I realized that would've been
so I turned it to wolves.
Makes sense as a beastie that expands into new lands by having baby beasties.
Makes less sense if I enchant, I dunno, a cleric with it ;)
This is a bit of a step up from Seed the Land, but that was pretty low power. This might be fine.
Why is this an Aura rather than a global enchantment?
Seems it's only happened once so far, Alpha Authority, though there's also Aspect of Mongoose. Both of those were untargetability + some other bonus for
, which this is as well, so your costing seems spot on here. (That said, I'm sure this would be perfectly printable at 
if you wanted to.)
I'm tempted to cost this at
but I'm not sure what green gets hexproof at an an aura.
I feel like Landfall's existence (my least favorite mechanic) means this card shouldn't exist.
Concept of red Overrun
Yep, that spasm can hit your own lands.