The Joker is not some sacred, impossible-to-touch character simply because Heath Ledger did a great job playing him in "The Dark Knight."
Far from it, actually. The Joker remains one of the most potent, richly-imagined villains in all of pop culture, and the notion of DC movies being made moving forward without any version of The Joker popping up at any point is, frankly, ludicrous. As long as you are telling Batman stories, you will also at some point be telling Joker stories, and that's fine.
However, there's no denying that Ledger's version of The Joker is an impressive one. The mere fact that it was so well-liked after Jack Nicholson's version had been anointed by pop culture back in 1989 was impressive. After all, you could argue that without Nicholson, that first film doesn't become the phenomenon it eventually became. I think 25 years is long enough for me to let go of my resentments about that version, and more than that, I think the real lesson is that The Joker can survive any reinterpretation.