The Joker was only 'off-limits' because Christopher Nolan did not feel it was appropriate to re-utilize the character following Heath's death; if someone did step into Nolan's shoes and do a continuation of the universe he established with the Dark Knight Trilogy, they would in no wise be beholden to the personal choice that Nolan made.
Anyone stepping into the 'Nolan-verse' would also in no wise shackled by what had come before except in that they would have to maintain a balance between striking out on their own and remaining true, spiritually, to what Nolan had established, which would not be that difficult a line to walk, IMO.