I don't know if this is legit(ripped from Anon), but it'll be fact-checked in a few days. Posting now so I mainly don't forget to check it.

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From the interview that was included in the April issue of the Bessatsu Shonen Magazine.

The series is still in production, but the series is more than half way through.

New contents will be included in S2 that we didn't see in the manga. [Isayama ask to include these stuff in S2 himself] because he wasn't able to include these things in the manga because of lack of time.

Key visuals and trailer will be release at a later time.

Araki is pleased with the effort the staff has been putting in on S2.

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New content that wasn't in the manga= filler. "He wasn't able to include them in the manga because of time" is a pretty nice excuse when your chapters are monthly released. Overall, don't really care. Manga is bad, anime is worse. Season two is delayed because anime only fags are going to be disappointed at the lack of fighting.