This concept of a "Mega Ultron" composed of thousands of Ultron bots was briefly considered for the climax of Avengers: Age of Ultron. Ultimately (and I think for the best) it was rejected, likely because as cool an image as it is it doesn't make a great deal of tactical sense over an overwhelming swarm of bots, and probably would have taken more bots than would have been realistic for Ultron to produce! The concept was cut before I was half-way through the image, but I was having so much fun with it I decided to complete it on my own time, so I guess it you could call it "fan art."