While Thanatics--"Thanos fanatics"--desperately want him to be built up past 2018, I simply cannot bring myself to believe it. There is too much evidence to think otherwise. Several CBM users have said they want both Thanos and the Infinity Stones to be built up until Phase 6! Y'all, that's just crazy. Like the title states, it is abundantly clear that Thanos will be the villain of the third Avengers film.
Here's why.
First, we need to make an educated guess at when Avengers 3 will be released. Since Kevin Feige has gone on record as saying they want a three-year gap between Avengers films, 2018 is the likely year. Both of the previous films will have opened in May, and since that's a hot month for blockbuster releases, it's safe to say that A3 is opening on the May 4, 2018 release date.
One thing that is heavily preached in the comment section of CBM articles is that Marvel does NOT work in trilogies! And yes, that is true. They take things one movie at a time. There is only the next installment. But people are taking Feige's words far too seriously. Even he said that Loki's three appearances form a "sub-trilogy." What I'm trying to get across is that they can adapt the story-telling aspect of trilogies to their movies. Take TDK trilogy, for example. The main antagonists of the first film, the League of Shadows, return in the third, and The Dark Knight is the bridge between the two. Similarly, Thanos is the reason Loki is able to do what he does in the first one, and both Thanos and the Infinity Stones would return in A3. But at the same time, A3 would lay the foundation needed for future Marvel movies and sequels.
Guardians of the Galaxy is said to definitely be connected to Avengers 3. But what is it about this film that makes it connected? Does it just mean that we'll be getting a GOTG/Avengers team-up? No, of course not. GOTG not only established the third Infinity Stone, but also showed just how threatening Thanos really is. Let's take a look at a quote from the brilliant director, Joss Whedon himself:
"[Thanos] is so powerful he is not someone you can just try out and punch him. Like he did in the comics, you want him to be threading through the universe and to save the big finale for 'the big finally'. He is definitely a part of what I have got going on. The thing about the Avengers is that they are very powerful but not very stable. So, there will definitely be some people that will shake them up in the next installment."
As we all know by now, Thanos will not be the villain of the next installment. But as we see here, he is a part of what both Joss Whedon and James Gunn have going on. Think of the state of the Marvel world by the time A3 comes around. Whedon will have been working on the Avengers and lots of other Marvel-related projects for seven years. Marvel Studios will have been teasing Thanos for six years. Do you know how much happens in six years? A ten year old who saw the first one will be sixteen by the third. As one site pointed out, six years is more than a lot of franchises last, let alone a villain/storyline.
Something a lot of people are worried about is that there won't be enough time between now and then to establish all the Infinity Stones, especially since Thanos currently has none of them. Marvel Studios have been building the Infinity Stone storyline since 2011 (even though the Tesseract wasn't revealed to be a stone until last year, we're talking about super-powered artifacts here--even the general audience will notice a recurring plot tool). Right now, we already have 3 confirmed Infinity Stones in two phases, possibly 4. And who's to say Thanos doesn't have one Infinity Stone already? The point is, there are seven movies between now and Avengers 3. That gives Marvel plenty of time and plenty of options on what to do with the "Infinity" side of things.
It also gives Marvel time and options plentiful to build up Thanos and his back-story. Besides that, Whedon has to make sure that a movie with Thanos will work on its own, without the need to see any previous movies. But for the sake of helping him grow as a franchise character, there are so many opportunities to include him. As the Red Skull is likely to return in Captain America 3, they could--and probably will--tie him into Thanos' story somehow, as it was an Infinity Stone that booted him off into space. Doctor Strange will be cosmic and magical, dealing with other dimensions and realities and worlds--boom, glorified Thanos presence, ala GOTG. Thor 3, very likely as well, considering the position Loki is currently in. I personally think we'll see him go against the Guardians of the Galaxy in their sequel, albeit only briefly. An Inhumans movie has been heavily rumored and confirmed to be in development, and since a lot of that happens in space, Thanos could very well fit there. Hell, they could put him in Ant-Man if they figured out a clever enough way to do it. Like I said.. Plenty of time and plenty of options. There's plenty of room for character development between now and Avengers 3.
As for the recent rumor that Avengers 3 will be split into two films, I don't believe it for a second. It's not that I don't think it's not possible, I just don't think it's likely. The article that was the source, from The Daily Marvelite, was very poorly written, and sounded more like wishful thinking than an actual scoop. But, that's kind of irrelevant to the point of this article.
Well, there you have it. Why I am sold on Thanos being A3's villain. Feel free to bash me or commend me in the usual place!