I had a revelation while watching AoU again...

In the scene where Wanda reads Ultron's mind and sees the meteor striking Earth, Ultron holds his dinosaur speech. The words that really struck me were :

"When the Earth starts to settle, God throws a stone at it. And believe me, he's winding up."

The whole mind reading part and the aforementioned quote from Ultron were really just him saying Thanos is gonna come and destroy everything, especially if you consider the D23 footage showed Thanos hurling the moon at Earth.