I'll add my guess. If the Space Gods are inclined to help us when they arrive, then a blanket of reality trees covering Maritime Southeast Asia would protect literally hundreds of millions of people from the bad guy invaders. Plus, the tropical setting would hearken back to their close-to-nature oTorg ways, back in the rain forests of South America. Finally, it would be a bit of an Easter egg for oTorg fans . . . the Akashans have a beef with Orrorsh now, and Maritime Southeast Asia was where Orrorsh was originally located, back in oTorg.
However, that would make Pan-Pacifica the only neighboring invader. It sets up an interesting bio-tech vs. bio-tech situation, which could be fun. Zombie breakouts in Akashan territory would be reminiscent of how the Akashans had to deal with the Comaghaz back then. (Plus, spaceships vs kaiju! Pew pew!). Problem being, the Akashans don't really have much of a motive to go after Pan-Pacifica, other than its vicinity . . . unless it's to chop through PP's holdings in Maritime Southeast Asia in order to get to Orrorsh's southern flank, and PP has the bad luck of being in the way.
Alternatively, if they're here solely for revenge, then putting down stakes in Central Asia would work. There's far less population there, but it puts them right up in the Gaunt Man's face, on Orrorsh's northwestern border, with no other invaders getting in the way.
So I guess my vote is
1. Maritime Southeast Asia, followed by
2. Central Asia