There is no definite premiere date for "Attack on Titan" season 2 yet, but a lot of plot speculations are already making the rounds. One of the most popular speculations involves Dr. Yeager and the secrets that could be lying in his basement.

Reports reveal that there are rumors stating "Attack on Titan" season 2 will be based on the manga series "Shingeki No Kyojin." Additional rumors also state that the first episode will be titled "Basement," which is also the same title of a chapter found in the same manga series.
In that chapter, Armin, Levi, Sasha, Eren, and Mikasa find out there are other survivors and that Bertolt and Hanji will lead the Survey Corps in the wake of Chief Erwin's death.

Other sources point out that Dr. Yeager's basement has been mentioned during the first season and that Eren has always wanted to go and check the place. In the first season, Dr. Yeager also told Eren that answers to the many questions in the world are found there.

Morover, it is said that the ongoing manga series, which featured the basement, shows books found in the area and that a photograph was found in one of the books' pages. Eren was injected by a serum that could be the reason why he can change form into one of the Titans. The photo could be a clue to the origins of such being.

It has also been noted that writer Hajime Isayama, has written two story arcs titled "Clash of Titans" and "Uprising" for the manga series, which will, in turn, be the basis of the anime's season 2 plot.

While the manga serves as the source for the anime, it is noted that several alterations are made to suit different mediums. For now, no premiere date has been announced, but it is believed that "Attack on Titans" season 2 will be shown in 2017.