6. An Evil Organisation
There have been more than enough organisations with evil intentions introduced in to the Marvel Cinematic Universe already and, in theory, any one of them could be looking to build an evil robot. A.I.M (Advanced Idea Mechanics), HYDRA, Hammer Industries or even the newly introduced Rising Tide in the Agents of S.H.I.E.L.D television series all have the resources to create something like Ultron and they all have enemies to give them a motive to do so. If not one of those listed, there are countless other villainous organisations - or even individuals - in the comic book universe that could be introduced on to the big screen who are more than capable of building a robot tough enough to take on the Avengers.