Sony Italy's Twitter account has confirmed that the company will be attending IFA and indeed it will hold a press conference on September 2 "to announce many new features." Interestingly, the Sony Germany account is mum on the subject.
Anyway, everyone expects the Sony Xperia Z5 to be the star of the show. The Xperia Z3 family was unveiled at IFA 2014 so there's no reason to break tradition.
Early rumors said it will be unveiled in September but in Japan, while "Made for Bond" teaser posters are doing cross promotion for Spectre (the new Bond movie coming out in November) and the Xperia [blurred]5 (the blur most likely concealing a "Z").
The new flagship is expected to grow the screen size to 5.5". The camera department will use the 21MP IMX230 sensor on the back and an 8MP selfie camera. The flagship should be powered by a Snapdragon 820 chipset, though that hasn’t been released yet, so we might see another 810 instead.
The screen size bump meshes well with today's unveiling of the Sony Xperia M5 – a bigger alternative to the Xperia Z3 Compact, with a sharper screen and a MediaTek Helio X10 chipset. It's too close to 5.2" screen of the Xperia Z3+/Z4, but will provide a good middle ground between the upcoming ~4.6" Z Compact and a 5.5" Xperia Z5.