If we talk open world in a new Batman game, I think it's important that the city feels truly alive. Arkham City had a great narrative excuse to have a segment of Gotham ruled by criminals and it worked greatly for me. But in Origins and Knight, though they had an excuse too, I think I just let it pass to enjoy the game, but with a bigger space in the open world, it wasn't as entertained to go through as it was in City. If a new Batman game decides to go the open world route, I would really appreciate that Gotham has citizens living in it and walking, driving, etcetera. It would be nice to see different neighborhoods, the narrows, the upper class suburbs, and so on. Of course, the Batmobile would be restricted to freely go through the city without concern on who you crash, but I think combined with a Batwing that the player can fly it would work fine. Still, I don't think every Batman game to come has to be an open world. Asylum was just fine with a smaller map, though of course roaming around wasn't as fun as gliding through the Arkham City
What I missed greatly in Knight were the boss fights. I don't think Asylum had great boss fights, with some exceptions, but they were at least entertaining and it was cool to face some classic Batman rogues. City and Origins had a great balance of boss fights and challenge that was lost in the last Batman game