The Avengers is filming in Downtown Cleveland and there is a ton of great videos and photos being leaked across the Internet showing there will be some form of alien invasion.
Whether its the Kree, Skrulls/Chitauri or whomever, director Joss Whedon will hopefully set up a sequel to feature the deadly robot Ultron in some capacity.
Obviously, there will be other stand-alone Marvel films between The Avengers and a possible sequel, but let me be honest in saying that every single person on this planet LOVES to see killer robots. If this statement isn't true, then why have all the Transformers films grossed billions despite the well-known hate for director Michael Bay and what he has done to the characters?
The reason is the Transformers are killer robots...well at least the Decepticons are.
Hopefully, you see my attempt to make some kind of point here and that Ultron should be the next villain in an Avengers sequel. (Okay, so Ultron is one of my personal favorites villains too, but again, who doesn't love killer robots?)
If you're going to have an Avengers ensemble cast and film, you better have a formidable force opposing them and having an invading Ultron army to take over would indeed be a formidable force for the Earth's Mightiest Heroes.
This is just my take on the matter and an attempt to look forward into the future of the film series.
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