Attack On Titan season 2, episode 1 promises to be very interesting. Season 2 will start off discussing Dr. Yeager's locked basement.
Dr. Yeager's basement has always been a mystery in the Attack on Titan anime. During the first season, reaching it was considered as one of the protagonists' goals, since Eren's mysterious father cryptically stated that answers to the questions of the world would be found there.
With Attack on Titan season 2 set to start its run next year, speculations are high that the theme of Dr. Yeager's basement would surely be discussed in the anime's first episode. This is because its existence, as well as its contents, are extremely pivotal to the story.
Considering the current plot of the manga, such notions were indeed correct, as the Survey Corps, after suffering through extremely heavy losses, was finally able to block the hole in Wall Maria and access Dr. Yeager's basement.
Inside, they do find a huge revelation, one which will shake the entire world of Attack on Titan.
Take note that after this point, there will be major spoilers.
Despite losing almost all of their comrades, the remnants of the Survey Corps found something very unusual in Dr. Yeager's basement. Several books were discovered, and while the manga has not yet discussed the contents of the book, the protagonists were able to find something very unusual in within one of the book's pages - a photograph.
It has to be noted that the world of Attack on Titan is somewhat based on the Industrial Age, where innovations such as electricity have not been discovered yet. In the Attack on Titan world, things such as photographs do not exist; thus, its discovery in the basement points to the notion that there is far more to the anime's world than what was depicted in the first season.
Grisha Yeager has always been presented as a man who was said to have come from outside the Walls. With this, and with the discovery in mind, it could be alleged that the doctor could have easily come from a community outside the Walls which is far more advanced and far more civilized.
With a revelation such as this, the story of Attack on Titan has just gotten a lot bigger. If any, the developments in the story could very well warrant yet another run of its highly-acclaimed anime.