Under the Red Hood is a comic series detailing the resurrection of Batman's second Robin, Jason Todd, after he's tortured to death by The Joker.
Todd re-emerges (thanks to the Lazarus Pit) as a vigilante who frequently clashes with Batman and, after being rejected by his former mentor, finally takes up the mantle of the Red Hood, which was formerly assumed by The Joker in his earlier days.
An R rating could really make the most of Todd's brutal, rage-fuelled physicality, painting him as a ruthless yet complex adversary like the Caper Crusader has never faced before, and certainly not in the DCEU. PG-13 probably wouldn't be strong enough to really make audiences feel Todd's anger in a palpably visceral way.
How Likely Is It?: It's sadly neither probable that this movie will get made nor that it would get an R rating even if it was made. There are so many other priorities for Warner Bros. for the foreseeable future, and though Todd's backstory could very well be rolled into another film, it's probably not going to get the fleshed-out treatment it deserves. 2/10