Hmm. There would be risk involved in returning to the Nolan-verse, and in bringing back the character of The Joker, but I don't think the risk would outweigh the potential reward to a significant enough degree to stop them from doing it if they so chose.
I do think that the risk associated with both ideas (returning to the Nolan-verse and bringing back The Joker) would be effectively and significantly minimized, though, by using the setup that Nolan provided to finally bring a variation of Batman Beyond to the screen.