User blog comment:B214/High School DxD Born Final Episode Review/@comment-15929029-20150624061950/@comment-26209278-20150624114424

Well, Rose becoming Rias' Rook was at the end of vol 7, after the events had finished, so it makes sense for that to be at the end. And the Great Red/Fang Blaster were teasers for the next season/season after, to keep us enticed.

Things shouldn't be too badly shaken for a possible season 4, since he will properly unlock the Triania in the first half of that, then fight Sairaorg in the 2nd half.