Rebuilt By Another Ally

It should go without saying that audiences didn’t see every little thing Ultron did during his short time on Earth, so he may well have some allies that we don’t know about.

Perhaps, for example, he built his mate from the comic books – Jocasta – and she may well be in a warehouse somewhere, missing him to the extent that she is rebuilding him in secret.

Or there could be another villain who was impressed with Ultron and wants to utilise him in the future – perhaps to create a live action Masters of Evil.

Maybe there are individuals who saw him as some kind of god (the Sons of Yinsen, perhaps?) and worshipped him to the point of wanting to bring him back.

Either way, Ultron may have allies out there that weren’t shown on screen and they may well have plans to bring him back.