Sensei's newest blog post has some information on P5V12.

There will be tons of added chapters from Rozemyne pov. So much that the book won't fit any SS chapters which is quite a lot, as the final volume will be half new content. Prologue will be Ferdinand pov and epilogue will be Lutz pov (which was also the last chapter of the WN).

It's both nice and a shame. Nice because Rozemyne pov is always interesting and it is going to be nice to get more of that. A shame, because the SS chapters are usually very good and give a different perspective to events. Let's hope we will also get other perspectives in a later volume that focuses on short stories, so we'd get the best of both worlds.