The Joker is a standalone film. However, as I mentioned earlier, they ARE rebooting Batman next year. 2020 release, written and directed by Matt Reeves. IF the Joker film is successful, and IF the new Batman film is successful, I think the two will combine after that.
As jammindude said, money talks, and at that point, if they have a legit success on their hands, they'll scramble to maximize the profitability. The DCU is in shambles, and everyone can see it. Fanboys can say all they want, but Justice League, and the whole convoluted stuff they did was in response to the MCU having so much success. DC got on board late, and then tried playing catch up.
This Joker movie, and the new Batman movie, are their attempt to reboot and do something different. Sorta how Deadpool is set apart to a degree, although they mixed in some X-men stuff.
The Dark Knight Trilogy was a standalone because Nolan refused to taint it. My thought is, DC finally realized they blew something great with the Nolan trilogy, and are now trying to do something different so they can be a bit more original. But time will tell.
Again, its ALWAYS about money for these studios. If The Joker and The Batman are successful, the studios will do whatever they can to maximize profit. It'll be interesting to see if The Joker gets an "R" rating. I hope it does.