That would be awesome. To have a faithful adaptation of Galactus and his Heralds in the MCU would be awesome. And it'd also explain who might have been responsible for decapitating the Celestial that became the location known as Nowhere.
As for future Avengers antagonists, I'd personally like to see: Galactus, the Celestials, the Beyonder, Kang the Conqueror, the Leader, and Maestro Hulk.