1. Seth Gable
Seth Gable, famous for playing Agent Lincoln Lee on "Fringe" and Count Vertigo on "Arrow" is my first choice for Mr. J. His portrayal of The Count on "Arrow" is very highly, and very obviously, inspired by Heath Ledger's portrayal of The Joker in "The Dark Knight". So, just slap some makeup on The Count, and BAM, Joker.
2. Cameron Monaghan
Monaghan played Jerome, who is supposedly a young version of The Joker, on the "Gotham" season one episode "The Blind Fortune Teller", and was creepy as any older version I've seen. This one might not work due to Monaghan only being in his early 20s though.
3. Johnny Depp
Let's face it, he can play anybody he wants and people would love it.
4. Aaron Paul
The former "Breaking Bad" star would SUCK as The Joker, I'm just saying ANYONE would be better than Leto.
5. Joshua Jackson
This other "Fringe" star played Peter Bishop. Bringing that character's conman personality to Mr. J would be a radical departure from Heath Ledger's (widely considered the best ever) portrayal, but one that would probably be widely accepted.