The New Imperiex?
Okay, over in the mini-series "Last Planet Standing" it seems that we finally know what Galactus is up to. He figured that since the Big Bang gave birth to him, a second Big Bang will kick-stark his evolution to the next level.
He's put in motion a plan to restart the universe, using the destruction of the Earth as the primer for the event. He's gathered a bunch of powerful items (including Thor's hammer, and Odin's Sword), contained a number of cosmically powered beings, and contained the mystical energies of Asgard.
The destruction of a galaxy in the first issue was a trial run of the process, now he's going to do it on a universal scale using the excess energies he's been gathering, and a new star that's appeared in space.
All this because Thor used Asgard magic to create a "special star" in the sky to honor the death of Captain America. And since the Spider-gril series is coming to an end, Marvel is free to end this anyway they please.