"You won't kill me because of some misguided sense of self-righteousness. And I won't kill you because you're just too much fun! I think you and I are destined to do this for a long time."
That kind of foreshadowing practically DEMANDS follow-up. I don't see how the Joker can possibly NOT be a major character in the next film.
It's not foreshadowing for more appearances by the Joker it's foreshadowing for the storied history we know that Batman and Joker have in the comics, TV, movies, etc.
It's a "wink at the auidence" about the legendary rivalry between The Joker and Batman not "foreshadowing" for more movies.
We've only a handful of movies in this Batman series and Batman has a pretty big and diverse rouges gallery. I don't think we should be doing repeats.
Leave the Joker gone. Maybe a chuckle in Arkham, a name drop, stuff like that but no story focusing on him again in this movie series.
Nevermind that you're not going to find *anyone* for the next movie to live up to Ledger's performance and whomever you pick is just going to just come across as trying to best/match it but it'll also tarnish Ledger's "legacy" with the character. -Not to mention the corniness of recasting characters in movie franchises.
The Joker story has been done, time to move on to Catwoman, The Penguin, The Riddler, The Mad Hatter, Ivy, Clayfast, Harley Quinn and so many others.