IA. Personally I don't think AC needs a spinoff any time soon. They went so big with it and expanding upon AA in so many ways. I say give it a rest and come bigger on the next gen with a true sequel that pushes the series forward in a way they can't currently do.
In the meantime Im all fog then branching out and doing a spinoff or prequel and there's so many routes they can take. Whether it be a Silver Age inspired game with influences from the JL, Batman Beyond or anything involving the Bat Family, I think there's a lot of room to think outside the box and bring us something we typically would see, especially if they were to stick with the Arkham line with their next title. I think people need to relax, be a bit more open minded and trust in Rocksteady's abilities bc so far they seen to know what they ate doing. Hey you never know; you just might like it