Of course, the first film was incredibly successful and saw Phoenix take home the Academy Award for Best Actor in the process.
Since then, the rumours about a second film have been swirling, but nothing concrete has emerged so far.
Now, Phoenix has at least speculated on where his character Arthur Fleck - a troubled comedian who descends into madness and violence - could be taken if another movie was to materialise.
Of his role in the 2019 film, he told Playlist: "This is an interesting guy, there are some things we could do with this guy and could [explore] further.
"But as to whether we actually will? I don't know."
So, that's not exactly what fans will want to hear, as there's no evidence to suggest that there actually will be a second film from the Joker-verse, but Phoenix's comments aren't the only suggestion that there could be more to come yet.
Specifically, comments about the director of the first film, Todd Phillips.
OK, bear with us here, because it's all a bit vague, but the theory comes from an article published by The Hollywood Reporter about powerful lawyers within the film industry, specifically their entry on Warren Dern.
In that entry on Dern - who counts Phillips amongst his clients - they said: "Todd Phillips struck a deal to co-write the next Joker instalment".
Again, it's not exactly set in stone, but there are encouraging signs for fans of Joker there, for sure.
In another THR report, in which they claimed that they were reported information from an industry insider, said that a 'planned sequel' is definitely in the works at Warner Bros.
That article, which largely focused on the forthcoming Superman flick, said that the second Joker film 'will take place in the same universe'.
That could certainly be interesting.
If the sequel can be made, it's certain to be a box-office smash, given the success of the first one.
It quickly became the highest ever grossing film to be rated R, and garnered 11 Oscar nominations, so if they could recapture that magic, and get the critics on side, there's no reason that they couldn't do that again.
Interesting stuff, to say the least.