Agreed, Jason Todd was one of the best resurrections in comic history. I would never go as far to say Joker ever dies, sure he fell from the top of a waterfall in Death of the Family, but no body was ever recovered, so its only a matter of time before he resurfaces.
As a prequel, you could be left with Joe Chill(who is widely believed to be Joker's true alter ego), or the original Red Hood(the character Joker was when he fell into the vats). There is also the fact that Batman has atleast a year of his career before Joker arrives on the scene, so he could very well be ignored entirely. There is a vast amount of rogues to select from, and with the over saturation of Joker recently, it is very plausible that Joker might not make an appearance at all.