The Dark Knight was a big deal because we hadn't seen The Joker on the big screen in 19 years. And in 89 we hadn't in 23 years. After The Dark Knight I was prepared to be perfectly content if it did take that long or longer to get a third cinematic Joker (2027-2031). And TDK achieved that with him having only 35 minutes of screen time out of a 2 hour and 24 minute film (not counting the closing credits in the run-time).
Now I've heard that Leto's contract signs him on to appear in 3 Suicide Squad films, 3 Solo Batfleck films, a Joker film with him and Robbie as the leads, and possible cameos in Justice League films.
If you're thinking "it's not a problem to see Batman in that many films" Batman was designed to be the main character. Yet some people do think he's overexposed, being in at least 1 Animated film every year, sometimes all 3 Animated films DC does. And now Batman elements are a core part of 2 Prime-Time TV shows. Gotham and Arrow which get numerous lesser known Batman characters plus Ra's Al Ghoul. And soon Teen Titans will have Dick Grayson and Oracle/Batgirl. A growing minority of people are getting sick of Batman.
And if you know me from my Internet activity elsewhere you might be thinking "You're ok with Lex being in every Superman film, and Dr.Doom in every F4 film, and Magneto in every X-Men film, and so on".
Well those are sagas that don't have the variety of awesome villains Batman has. And those Nemeses are also more human characters, who have personal relationships with the heroes. Each has at least in some depictions been a Friend first. There is no Frenimy vibe with The Joker, no teaming up against a greater threat, in TDK The Joker is the greater threat that causes Maroni to help the good guys.
But beyond that The Joker is not a run of the mill villain or Arch Nemesis "When Supervillians want to scare each other, they tell Joker stories".
In terms of what seeing The Joker show up in a Batman story should mean, I don't consider him comparable to Lex Luthor but rather to Brainiac.
Harley Quinn being in that many films I'm fine with. In fact I think it may hurt Harley's character that Robbie's version of her will never be seen without The Joker. Her relationship with Ivy is something I definitely want to see. And Harley herself can be a reminder that The Joker exists without actually seeing him.
Even seeing The Joker is every Batman film of the future series I feel is bad, because how will other Bat-Villains truly get to shine if they're always in the shadow of The Clown Prince of Crime? Lex appearing with other villains is different because he is designed to be (the CEO version anyway) an arc villain, The Joker is the Wildcard that characters like Lex would want locked up just as much as Batman does.
But beyond that, is the idea of The Joker being in non Batman films. I was iffy on him being in Suicide Squad to begin with. Assault on Arkham works because those Animated films are supposed to be outside the box experiments. Introducing the Suicide Squad to non Comic readers with The Joker's shadow hovering over them shows a certain lack of faith in the property, if Guardians of The Galaxy didn't need Loki then SS shouldn't need The Joker. If however for all the hype Leto winds up being in the film no more then Thanos was in GotG then I might consider that a relief.
But now that it sounds like he may be in all 3 Suicide Squad films? How does this even remotely represent the kinds of stories the Suicide Squad usually deals with? They usually don't even operate within the US. They should be like Mission Impossible but with a team of villains.
And a Joker solo film...... yes there have been Comics with The Joker as the Title Character. Their appeal has always worn off pretty quickly.
A film with Harley and Ivy as the leads, that could work. But ideally of characters who are Batman antagonists it's Catwoman who is best at carrying her own stories. But of course Executives think the Halle Berry film failing means Catwoman can't carry a movie. Rather then simply blaming that film's failure on being Catwoman in name only.
The Joker in a Justice League film could work maybe if they do a Legion of Doom story, but the LoD has never existed in the actual Comics, closest analogue is the Secret Society of Supervillians from the Infinite Crisis lead in. Infinite Crisis issue #2 explains why The Joker wasn't asked to join that. So I wouldn't want The Joker as a LoD member but as a Wild Card.