Every video to be shown publicly needs to receive an age rating and a rating for a Diablo 3 Expansion trailer has been posted on the German ratings board site USK. It gives the trailer a rating of 12.
Update: As there’s some confusion in comments, note that this rating applies only to the content of the video to be shown publicly at Gamescom. This rating tells us *nothing* about the content of Diablo III: Reaper of Souls, which will in all likelihood be Mature, the same as Diablo 3 and all previous titles in the series.
Blizzard’s public Gamescom 2013 schedule begins on Thursday but they are scheduled to hold a press conference for media tomorrow where the Diablo 3 Expansion, Reaper of Souls is expected to be revealed. That or a new wallpaper.