The Clown Prince of Crime will return to take another film on his own. 

The author and director of Joker, Todd Phillips posted a photo on Instagram of the cover of

the script confirming that he is currently working on Joker

This seems to confirm that Phoenix will be back to resume his role in following the 2019 film. 

Another comeback to follow is a poem by another Phillips author Scott Silver, 

Another unknown about Joker 2 is what the plot might be. 

Folie à Deux is a psychological term that refers to a mental disorder that affects two or more people, usually in the same family. 

Based on the plot of the first film, and that Thomas Wayne could be the real Joker father, 

it is entirely possible that the movie might introduce Batman and link him to the Joker in the opinion of Folie à Deux.

Yes, to know exactly what the movie will be about, we will have to wait and see.