We saw him at the end of The Avengers, now recently his plan came to light in Guardians of the Galaxy to obtain the infinity stones. We have come to know Thanos as the Evil Overlord of the Marvel Cinematic Universe, having orchestrated its main events from behind the scenes. But given his major setback as Phase 2 comes to a close, The Mad Titan is more than likely to come fourth and take action personally...
Now where does he start? I mean, as far as we know most of the stones are out of his reach and safely in the hands of his enemies, and he is running short on villainous subordinates to do his dirty work.
Or is he?
With quite a few movies to come before Avengers 3 we'll have a look at how they can tie in to the endgame of Phase 3, with sub-plots evolving into main ones, unexpected twists and events coming to fruition in accordance to Thanos' design as he reclaims the stones. Perhaps with the help of a few familiar faces...
Okay, I think this one can be left out. Ant-Man could just be a stand alone film with no ties to the Infinity story line and only hints of events previously seen or upcoming in the MCU. Though you cannot rule out he will end up making a big contribution to initiate Phase 3, not matter how small he is.
Captain America 3
If anyone remembers in The First Avenger, the Cap's War time nemesis, The Red Skull, vanished after holding the Tesseract, to an unknown dimension. Now over 70 years later, how will he worm his way back into the movies? An idea could be that he came across an infinity stone/gem himself, coincidentally the time gem.
With it he kept himself young, its the only logical explanation (Since both Captain America and the Winter Soldier cheated time with being frozen, that might not be an option for the Skull wherever he is). Secretly expanding his Hydra operations in space, he intends on returning to Earth with an army and advanced weaponry capable of mass destruction. Added with the gem, he could become an even greater threat than before and cripple the planet even before Thanos gets his hands on it.
If that sounds too crazy, he could just well be working for The Mad Titan himself. After all, he was originally set to appear in The Avengers alongside Loki, and the animated series Avengers Assemble have already entertained the idea.
Another way for Thanos to claim an infinity stone.
Guardians of the Galaxy 2
This is probably the most obvious prediction that came to our minds after the first GOTG; Ronan's betrayal of Thanos left yet another stone lost to him, now sealed away on Xandar by the Nova Corps. It will undoubtedly be his main target, as the orb appears to be the most powerful of the six, rivaled only by the Aether, currently in possession of The Collector.
It seems only fitting that he will get his hands on both by the end of the movie.
The sequel can also introduce Adam Warlock, a key character in The Infinity Gauntlet story line, to the MCU. He will be revealed as Peter Quill's father (If it's not J'son of Spartax, of course) and a main character for Avengers 3 (Director James Gunn confirmed he was inside a cocoon in the first movie).
Now, this is when things come together to build up the intensity that will bring us into Avengers 3. It would seem that Loki has fooled everyone, EVEN US!!!
How? Well it all goes back to the first Thor film; We know he wants power, and the throne of Asgard is the key to this goal, it always has been for him. Now why, after failing to claim it the first time round, would he change his mind, make a deal with the most powerful being in the universe, and decide to rule one of the weakest planets of the nine realms? (No offence)
Simple: Loki never wanted Earth, he could have taken out the threat of all the governments and target their military bases rather than spread the attacks around New York, a location that was deemed 'nukeable' by the world leaders anyway. Evidently his plan was one-dimensional, flawed and stupid, and that was actually the point. The god of mischief simply used the Chitauri invasion of Earth as a front so that its outcome would lead to his capture, imprisonment, and when the time came, his infiltration of Asgard. All under Thanos' command.
Of course Loki would have mentioned him in The Dark World if he really feared for his life; The Other made good with those threats on his failure. But Thanos well knows the Tesseract is now in the hands of his ally rather than S.H.I.E.L.D, and of course, the Infinity Gauntlet is in Odin's vault; their deal may have included these in exchange for Loki retaining the throne he always wanted. That was the real plan all along...
Avengers: Infinity War
While the plot for Doctor Strange is still being developed, he could be in possession of at least one of the infinity stones/gems, let's say the Soul gem. If that is confirmed then there is a tangible guarantee that Avengers 3 will have Thanos attack the Earth in a bid to claim it, along with the mind gem, revealed to be in Loki's scepter (He gave it to Loki so that he could establish a link with the Tesseract in the first film, explaining how he managed to teleport there in the opening). Unless he obtains it in another cameo in Avengers: Age of Ultron, this will more or less happen.
Having acquired all six infinity stones plus the gauntlet, Thanos leaves the Earth ravaged and proceeds with his Universal conquest, prompting the Avengers and every hero introduced in the series so far (Maybe even cameo appearances that lead us into other movies, e.g Black Panther) to stand against him. No doubt this will be much similar to The Infinity Gauntlet arc, resulting in the demise of The Mad Titan and all his chaotic malice undone. As the heroes triumph it will seem like an appropriate ending for the franchise.
Of course a post-credits scene could have Death resurrecting Thanos like in the comics, implying he is not going anywhere, not at least until Avengers 6, depends how far the studio wants to go. A compacted multi-movie trilogy (Meaning Avengers 1-3 with the rest in between) or a saga. Only time will tell.
As seemingly fun as it is to write scripts, these are, at the end of the day just theories based on what I have seen in the MCU and know from the comics. The writers probably have other ideas in mind, hopefully even better ones. Anyone else got their theories?