I could see a world in which this movie is thought to connect to Leto's Joker.
Because...
Or maybe audiences will think they are recasting, and Phoenix is now the new Joker.
And with a vocal majority condemning the Leto Joker, this is a problem because...
Or they might find that they prefer this Joker and resent having to go back to the other continuity.
This assumes that the other continuity will heavily feature Joker, or that the other continuity won't present something worth going back to if they do feature Joker. We don't know that at this point.
Or they could use this movie as evidence that WB is all over the place with DC, and there's no real point to see it in theaters. That's what drove me away from X-Men franchise.
I don't see how "making movies about DC characters" equals being "all over the place", but...ok.
There's no one approach that is going to thrill all audiences or get everyone into theaters. There are going to be people who like some of WB's approaches, and hate others. That's sort of the point. With this film, WB can reach an audience that perhaps they might not with their more mainstream stuff.
If it's an Elseworld tale... fine... but they better market it that way. Otherwise, folks will get confused about how this fits, or worse, give up after the appearance of it not fitting.]
It's essentially been marketed that way since the first news and rumors of the project.
Again, you could do this movie at any time.
And they're doing it now.
Wouldn't it make more sense to make an Elseworld Tale after you've gotten your Primaryworld Tale already established?
No, because this move is going to be its own, likely self contained story, and is likely going to play against what people know about The Joker from previous adaptions, films, etc, and the larger pop culture, not be reliant on what people know about the specific portrayal seen in the DCEU.
Beyond that, the primaryworld tale kind of is established. By the time this comes out, there will have been six DC Universe films. By any reasonable standard, that's a pretty well established franchise.
This situation reads as a movie that wouldn't be made if a top billed actor wasn't interested. But is that a good reason to make a movie? Cause one guy wants to play the role? What about the plan? Let's stick to the main course before we get to the dessert.
I don't see why that matters. Lots of movies are made as star vehicles, or because of some combination of star power and an interesting story.
I doubt that's the only reason that everyone involved in the project is making the movie, though.
You're assuming that this wasn't part of a broader plan regarding the DC characters to begin with. Which is just that...an assumption. Regardless, it's clearly part of the plan now, so I'm uncertain as to why it would be an issue that it may not have been part of the plan at some arbitrary point in the past.