So, wait, what exactly are God Hand's goal?

[Franckie]

Regarding the God Hand, at least Void is implied to have his own hidden agenda. When asked about the ultimate goal of the God Hand in an interview published for the Berserk Illustrations Files, Miura mentioned that it's too early to reveal the God Hand's goals but gave a hint to speculate over: "The key word is Void".

Well, Guts talked about Griffith's goal in Chapter 345. He just wants to keep flying higher and higher. He doesn't really need to enslave people or eradicate the others, it's not like they are a threat to him. He's keeping his followers in his "perfect kingdom" for himself. No sane man would want to leave such a place.

I am curious to see what is that "Age of Darkness" he will bring because it implies that even the people of Falconia won't be happy.

Merging the Astral World with the Material World flooded the world with monsters and caused monstrosities like the Sea God to reawaken, transforming the planet into a death world for humanity, save places like Falconia and Elfheim. This is probably what the "Age of Darkness" means.

Griffith turning Falconia into a hellish place, let alone tolerating someone else trampling his kingdom, seems out of character for him. From Griffith's point of view, Falconia is his, the land, the peoples, his to do with as he pleases. He would not tolerate any interlopers despoiling his property. In a twisted sense, Griffith is a possessive sheepherder who looks after his flock of apostles and humans.