With Marvel lining up movies for next century like ordered pizza, no wonder the fans are being driven insane by the awesomeness of it.
So, what is the most probable theory? Its impossible to predict. But speculations are given a seat. With Civil War releasing next year, followed by [Thor: Ragnarok](tag:956858), let's refer the 'real comic history'.
WARNING : MOVIE SPOILERS AHEAD
No doubt Tony Stark is really another milestone of Science and Technology, everything he touches leads things from bad to worse. Latest being 'Peace in our time' (say, pieces in our time) A.K.A. Ultron!
"Ragnarok is a fictional character, a supervillain who appears in comic books published by Marvel Comics. He first appeared in Civil War #3 (July 2006) and is a cyborg clone of Thor."
He was created by Tony Stark (not him again) to bust the anti-registration heroes - Cap'n and Team. But the dude went out of control and savaged a great deal. This may sound like another Avengers storyline. But It has way too much to do with Spider-Man series and X-Men series. And a lot of super heroes never really appeared in past MCU creations.
Now, If its really going to be the Cyborg, then In Civil War, we should assume, that things won't end well (I mean...won't end like.. they happily lived ever after).
Our Ragnarok may make a mess out of Asgard, killing a whole lot. Maybe even Thor! Or the rest and Thor may stay at the ruins of Asgard. This will satisfy the translation Ragnarok = end of all things.
This way, you can assume an awesome fight between the real Thor and 'Thor dupe'. May be this is why Civil War was scheduled before Thor 3. (Or may be ILM pleaded on their knees to give the VFX team some rest :)
The current condition of Tony Stark being 'completely imbalanced' adds support to this theory.
What are alternate theories? On the other side, we have - Loki, Surtur and of course Thanos.
But let's make it sure by 2017.
Will it be Cyborg Thor?
If you are not entire a Marvel kind of person, ExtraChameleon head to for something else.