It's happening! The thing we've all never asked for but deep inside have wanted since the earth was formed! Marvel Comics and the popular anime Attack On Titan are getting a crossover! Not much is known about the crossover event, other than that it will premiere in Japan in 2 weeks! Unfortunately nothing has been said about a US release, but with Marvel's fanbase and Attack On Titan's rise in popularity (due in part to Netflix and the Adult Swim block Toonami) it wouldn't be much of a surprise if we got the manga soon after Japan does.
For those who don't know; Attack On Titan follows the story of Eren Yeager, who lives in a post-apocalyptic world where humanity must battle against a race of gigantic monsters known as "Titans". Eren enlists in the army after his mother is devoured by one of these sickly looking human eaters! Pretty soon afterwards, he discovers that he has the ability to transform into a Titan!
It's unknown if this crossover will be canon, since the AoT manga series is still ongoing. I doubt it will, but it will still be an amazing crossover event regardless of whether it is canon or not.
Much can be speculated about this, but for now, anything anyone but Marvel or the creators of AoT says is just rumors or speculation. You can bet there will be a bunch of Marvel Heroes VS Titan battles though! And here's hoping that Venom and Galactus become Titans! That would be amazing.