2. The Only Way To Bring Bruce Wayne Back
If the next film is to continue in the Nolan thread, with Bruce Wayne retired, and Gotham in the hands of Blake as protector, the Joker is the best excuse to bring an older Bruce Wayne back to Gotham. Only a catastrophic event could have Wayne change his mind and turn his back on peace and self-exile, and while some will suggest a major character death or kidnap would be enough, the finality of Wayne's decision to leave at the end of The Dark Knight Rises suggests otherwise. That finality could only be reversed by the reappearance of the Joker. Who is to say that the Nolan universe strand needs to return in the next few years? A JLA movie will at least temporarily satiate the thirst Batman fans have for a new film, and the sense of finality that Nolan sought with his Dark Knight trilogy affords whoever takes over the reigns the luxury of time to evaluate the situation, and to either reboot or continue at a future point. That second option would be personally preferable, with an older character actor brought in - like Mark Hammill perhaps - who could convincingly offer a new take on the character at a different stage in his life, and who could retain the threat that the character brings to the table.