User blog comment:B214/Light Novel Volume 16 spoilers/@comment-162.202.198.72-20131026072404/@comment-1359322-20131214102420

Not really possible, a flat chested Rias is a nightmare. Not only to Issei but to the fans too. At the end of the day, we don't even know if 666 is going to be successfully unsealed or not. Cause it's a known fact that if 666 is truly a beast on par with Great Red, then only Great Red can and should defeat him.