I love Griffith, fantastic fictional character, but his motivations aren't all that complex. He's pure unbridled self interest and ambition. He has his mythical utopian kingdom now, but as Guts explained to those wizards, Griffith will continue to seek even greater heights, because that's just the kind of person he is. Greater heights, greater ambition, greater perfection - it's all about the climb for that dude. And that's why he was able to sacrifice the Hawks. What makes him interesting early on is that despite his almost otherworldly charisma and talent, he has flaws that make him vulnerable and eventually lead to his downfall, and those very same flaws are part of how he got where he was. Narcissism, perfectionism, a possessive nature. If his taking of Casca and fuck you to Guts is any indication those are still in play but he's so aloof and unstoppable lately its hard to tell.
My theory is his next goal is subjugation of the other Godhand, who have also incarnated along with Fantasia. That would be interesting to see.