According to Cosmic Book News, Thanos creator Jim Starlin confirmed to fans at the Montreal Comic-Con that the character will appear not only in Guardians of the Galaxy, as Marvel Studios President Kevin Feige has hinted, but also in the untitled sequel to Marvel's The Avengers, as has been widely speculated.
"Armand Rock attended the recent Montreal Comic-Con and spoke with Jim Starlin who said that the deal is already in place," reported CBN. "Jim Starlin even mentioned that Thanos will appear in the upcoming Guardians of the Galaxy movie in 2014 and The Avengers 2 in 2015."
Many had speculated that when Starlin reportedly met with Marvel at the beginning of the month that it was an attempt to repair the fractured relationship that the publisher has had with him for years, which would presumably clear the way for using his characters without fearing he might attempt a lawsuit a la Gary Friedrich or the Kirby Estate. Marvel won both of those cases, but not only were they a noisy inconvenience, but comics industry gossip is that Starlin had a better deal in place and might actually have a legal case should he decide to pursue one.
For his part, Starlin said at the Albany Comic-Con earlier this year that if he were to work with either of the two major superhero publishers in the future, it would likely be DC, because Marvel had mistreated him and failed to meet their contractual agreements with regard to royalties on reprints of Starlin's old work.
Magus, a Starlin-created villain who was a frequent nemesis of Adam Warlock, is also rumored for Guardians of the Galaxy, as is Warlock himself, although Warlock is one of the only members of the Abnett-Lanning Guardians of the Galaxy team not yet confirmed to appear. It doesn't seem particularly unlikely that Magus could be the central villain for the film, working under Thanos in the same way that Loki did in The Avengers.
Starlin kicked off a furor among Marvel fans when he told Cosmic Book News back in April or May that he had never been told by Marvel that Thanos was to be featured in the film adaptation of The Avengers, and neither was he invited to any screenings of the film. It would appear that under the guidance of Joss Whedon, a self-confessed fan of Starlin's work, whatever policy led to that circumstance may have changed.