It was always planned as being a stand-alone film.
One and done.
What else do they have to explore?
How the Joker is able to get so many followers/henchmen, even when they see how they are treated by him?
How does the Joker get the skills/knowledge to make his joker-toxins? How does he get his immunity to his own toxins and toxins in general?
This they can all do BEFORE the need to tie into the main DCEU (or the Batman character at all).
And in the comics, they are currently talking about there being THREE Jokers. Are they going to touch on that?
Also in the comics, they had talked about the history between Batman and the Joker goes way way back. (And not as that farce gotham high), but with the Joker knowing Batman’s identity almost since the start, yet up until “Joker War”, never used that knowledge directly against Batman’s financing. Sure they had that one story arc where the Joker attacked the Bat-Family as deminishing Batman legacy (was in the batcave and in wayne manor – kidnapping Alfred, etc.)
I do hope they leave it un-connected to the Batman and the DCEU in general.
I don’t know if I want Joaquin Phoenix’s Joker to be the next version to go against Batman.