Now a Lego Set will most likely not say explicitly what will happen in the film. However, it will respect it. In this case, Scarlet Witch seems to come to Hulk's rescue, which has a defensive stance when compared to the massive armor. Furthermore, Ultron's stance is very suggestive.

Check it out:

Is Ultron controlling the Hulkbuster?

Ultron is known to control all types of machines and technologies. Furthermore, his aim is to exterminate all humans on Earth "“ so enslaving the Hulk by using Scarlet Witch's powers doesn't add. However, he may have taken control of the colossal Hulkbuster, especially since he knows that the suit was especially built to stop the Hulk.

Where does that leave Tony Stark? Of course, he may not even be in the suit. However, his Iron Man suit is also susceptible of being controlled by Ultron "“ which also leads us to believe that Tony Stark is a prisoner in his own armor (much like what happened to War Machine in Iron Man 2).

How does that sound to you? Is it plausible? We believe it is.