So, I was thinking... which one of these would be more interesting?
Their Coming Is At Hand
The Earth is ancient, both in body and in mind. Mortal life has not always ruled it, and their age is now at an end. Its old masters, the Titans, are awakening, terrifying giants kilometres in size, carrying mountains - or even cities - on their backs. But they are not the only beings looking to master the world - from the abyssal depths that connect all worlds, the Deep Ones rise, singing dreams of prophecy and destruction, bending man's will to their own and bringing madness wherever they go. From the black void between worlds come the Star Gods, grown fat from devouring stars, alien and incomprehensible to man in their goals and thoughts. And beyond, even stranger and more ancient beings, turn to Earth with hunger on their minds.
This all was foreseen. Forces and invididuals from the dawn of history have long prepared for this day. Beneath the illusion of everyday normal life, ancient conspiracies and prophecies of doom form a network of defense, employing the Enemy's own weapons against it. Ancient-mortal hybrids fill their ranks, and slumbering beasts bound to the will of mortal bodies are their potent weapons. But will these halfbreeds and imprisoned gods retain loyalty to a dying race when their true brethren arrive? The ever-advancing technology of mankind is a potent weapon, but will it suffice against the power of living gods?
In this, players would be hybrids and people with Ancients imprisoned in their minds. However, there would be Enemy Slots that 'guest players' would make and then play when I decided their return was at hand. I think this would be easier/more fun to GM and story-wise run.
The Age of Awakening
In this, players could play both the returning/waking Ancients, or the Shackled (hybrids, imprisoned, avatars of ancients) - however, being on the humans' side would be actively discouraged. Only the Titans of the ancients would see humanity as anything more as food, and even they'd consider them a dying race and mostly useless. The Shackled would have to fight their Ancient sides all the time, risking losing control at the worst possible moments if they try to keep their humanity.
Or, a Compromise; some players take hybrids/imprisoned ones, some players take Titans, and both defend the Earth against the Deep Ones, Star Gods and whatever unique beasties might attack.