Ledger's Joker was, quite apart from anything else, very definitely the Joker. However, there's a school of thought which holds that Leto's Joker isn't the Joker, but a pseudo-Joker. Whichever way you look at it, that's a fairly hefty shift from the norm. Basically, the Joker we see in Suicide Squad is actually one of Batman's former companions, a young lad by the name of Jason Todd. He was the second man to take on Robins cape and who was killed in an explosion by the Joker (there was a phone vote to decide poor Jasons fate, which the Jason dies option won by a mere 72 votes out of 10,614). The theory is that the J under Letos eye either references the Batman: Arkham Knight video game, in which Jason is branded with a J in that spot by the Joker himself, or that the J stands for Jason rather than Joker, a constant reminder for Batman that he failed Jason whenever he looks at him. Add to that the display case which shows off Robins suit in a very lest-we-forget kind of way in the Batman V Superman: Dawn Of Justice trailer, and the taunting graffiti scribbled across it in the Jokers handwriting, and it starts to look semi-plausible.