Let's put the speculation of who the Great Destroyer is in here, as well as how the Incursions started.
Here's something to think about:
A little while ago there was a Fantastic Four #600 came out about the return of the Human Torch. A little known extra story that Hickman put in there was called, " The Arc", and in it, Galactus had sent his Herald to fetch Reed Richards to bring a device back to Galactus on the moon. Here, Galactus states, the Asgardains have the Galactus Seed hidden in the World Tree on Earth. This seed is a Celestial mistake, an aberration and he has to destroy it, (if you can believe him). What Reed has to do is when the Earth is about to be destroyed, (and that's a bad thing), Reed should activate the Arc, (the device Galactus gave Reed), and Galactus will come and destroy the seed.
Then we come to Uncanny Avengers #017, and the Earth is already destroyed, but Reed never activated the device to fetch Galactus, (who happens to be in the Negative Zone right now). So Galactus couldn't destroy the seed, and what happens is when the Earth is destroyed, (in an alternative Earth like UA), it will release the anchor to the seed, and it will tumble through space-time to places less accessible. We could have seen the first Incursion with Uncanny Avengers #017. But then we'd have to speculate on what an Incursion is defined as.
But why the Asgardians would have a Celestial seed is the mystery. Is this something Odin had to plant under the World Tree when he lost to the Celestials in Thor #300? And then the Mad Celestials of the Doom arc in FF, are storming around the multiverse as well. Is Hickman going to tie that FF story arc to the New Avengers as well?
Intriguing?