Perhaps Heath Ledger is still with us. Perhaps Christian Bale is out of acting. (No fan of either of them.)
Ledger would definitely still be alive. him dying by overdose is probably misunderstood by alot of people, making them think he was a junkie, when that really wasn't the case: both he and Bale are method actors, so when Batman is fighting Joker at the end of TDK,
Bale is actually physically beating the shit out of Ledger. those aren't stage punches, he is actually being pummeled to make the whole thing more convincing. that's why he was on the painkillers in the first place, leading to his overdose and death. (as an aside, i wonder how Bale himself feels about all that?)
going further, Bale probably wouldn't be as big a name without his role as Batman. assuming Terminator Salvation is still made, he may not play John Connor.
another one may be that the Arkham series of video games is butterflied. Batman will always be popular, and Arkham is based very much on both the comics and the animated series (even having Kevin Conroy and Mark Hamill reprise their respective roles) but without the Nolan series, there may be less incentive to produce the game. even if it is made and is as good as it was IOTL, one thing would almost certainly come to pass: the Joker would look different, as he's a composite of pretty much all previous Jokers. in particular, he'd probably lack the glasgow smile that Ledger's Joker has, which in my opinion really works to truly portray the Joker as a psychotic clown. here's a picture of Hamill's Joker from the animated series:
he looks very much like a clown, but here's another pic of the same character:
while the Joker
usually has a smile on his face, with this general getup (and even considering him having a physically modified face in some versions so that it's fixed in a grin (like Nicholson's Joker) the glasgow smile on Ledger's really sells him as a clown even though alot of times he's frowning (even considering that he, too, has a scarred face which technically means he's always smiling)
looking at Ledger's face here, you can see that he's frowning, but the makeup really makes him look more like the classic clown where, regardless of his actual expression, he appears to be smiling. for a bit more context, here's Pagliacci,
also with a glasgow smile but
very sad