I've said it before, but I think the most interesting non-Batman approach for Joaquin to do a Joker "sequel" would be to create an entirely new/different origin for The Joker -- not Arthur Fleck. Different story, different origin, different time period/location setting, different tone, etc -- but with Joaquin starring as the titular character again and Todd Phillips directing. I think this could work well for a variety of reasons that would benefit us (the viewers) and the Phoenix/Phillips team.
I loved this Joker movie and found the Arthur Fleck character fascinating. Loved seeing his transformation into the "Joker". The most exciting next step would be to see this Joker come to blows with some version of Batman. However, as we all know, that doesn't seem likely to happen any time soon. While Joaquin has said that he is open to doing a sequel with Phillips, he has also said that he doesn't have much or any interest in going toe to toe with Batman. We also have the new Reeves/Pattinson Batman franchise gearing up right now that will last several years, and we know WB likely won't be allowing multiple live-action Batman films/franchises to be happening simultaneously. It also seems HIGHLY unlikely that WB would be willing to put Batman in a disturbing hard R-rated film at this point.
Assuming Batman is currently off the table for a Joker sequel, that would leave us with following up with Arthur Fleck/Joker after the events of this film. I personally thought the ending of Joker was pretty perfect, and perfectly ambiguous as to whether some or all of the film's events really happened. Seems like a follow-up story would sort of defeat the ending of this film and miss the point of what the film was ultimately going for. It would be like making a sequel to Taxi Driver, or King of Comedy especially.
So what would excite Joaquin enough to play the Joker again, without him feeling like he's simply repeating himself or cashing in on the first film's success? Telling a completely different origin story that still captures the essence of the Joker character. I feel like this would be even more of a challenge for Joaquin/Phillips than the first film was, it would fit with the ambiguous "one possible origin" concept that the first film had going for it, and would certainly fit the "multiple choice origin" concept that has been associated with The Joker for decades now. It's also another way to tell a Joker story without needing Batman himself to be involved in a major capacity.
I know this will likely never happen and that we're more likely to get a straightforward sequel (if we get one at all), but I think this would be a really cool concept and would continue to show how infinitely The Joker character can be re-interpreted and re-adapted while still keeping his core elements intact.