Didn't see a thread like/for this anywhere else, thought I'd take a crack at this. Don't think its too spoilerific, just drawing from existing lore and inferences.

Throughout the main storyline I wondered what Gaius's endgame was and his obsession with conquering Eorzea, something commented on by several characters throughout the game. Three attempts across fifteen years from several different angles, ultimately going it alone on the last attempt. What is it about this one region that demands his brand of leadership?

The situation in Garlemald seems to be tense at best, with the emperor rumored to be on his deathbed. Whether Gaius received his blessing to take another shot at Eorzea is never really established, though he was said to have become disillusioned with taking it after the fall of Dalamud. Along with the strategic value of the region, with its abundant aether, Gaius seems convinced that the only way forward for Eorzea and its people is under Garlean management.