I've been toying with this idea for a while, and I'm keen to hear what the rest of you think.

Many people hold the idea that it will be the Giants that summon Shiva, but I believe that is off the mark. I believe the Giants will play a more significant part in unfolding the story of the Allagan Empire. In fact, I suspect the giants are the race of the Allagans.

One of the sidequests around Saint Coinach's Find revolves around the giant's fascination with the crystal tower, something they hold as sacred, as well as any depictions of it. Also consider the Flames' levequest, "Who Writes History".
Gigas converge on the Crystal Tower, led by a firebrand calling himself Okeanos the Red. He incites the mob to take back Silvertear Falls and reconsecrate the ruins of the Allagan Empire, from which he claims descent. Reunite Okeanos with his ancestors, and let any who stand in the way anoint the ground with hot fear and cold blood.
In the fight, he says the following:
Okeanos the Red: MY PEOPLE CLIMB TOWER AND RETURN TO HEAVENS!
On top of this, Acheron himself is described as an Allagan hero, and the guy is huge. Like, Giant-breed huge. To top it all off, NOAH comes to the conclusion that whoever built the Labyrinth of the Ancients "clearly thought bigger was better".

Or maybe just giant-sized?

Yeah, what do you think?