1: the first part of the sequel to involve batman chasing down "joker", capturing him, and incarcerating him in arkham ..... with a big chase sequence, like at the end of BB!
2: let "penguin" and "two-face" be the main protagonists ..... against EACH OTHER ("penguin" being the main villian, and harvey on the side of the law! until THAT little acid-on-face kerfuffle 😊 ) with batman helping harvey dent
3: dr harley quinn, or o'quinn, comes into it as a specialist, to deal with "joker"'s spiral into madness! at the end, she says that he ISN'T crazy, and reluctintly, let him go ..... and as he frolics outside, with his new squeeze ..... HERE COMES SCARECROW to mess him up permanently!!! she screams, he goes royally nuts, scarecrow falls off his horse, and batman? not very happy ..... CUE THIRD FILM!
4: NO catwoman, NO riddler, NO clayface (sorry, too similar to the sandman in SM3), and NO "joker" shooting barbara gordon (leave that for the third, maybe as the first bad thing that a SECOND "joker" does ..... a-la THE KILLING JOKE!)