So
this one's more sketchy than most because it involves a lot of speculation, but dataminers claim to have found this line of code in the last update:
"/Jack6_C06_v01_070925.mb_Jane1_pony_hairSystemShape2.mchp""
Which when read in the context of how the developers name and store data within the game shows that the character 'Jack', i.e. Joker, seems to have an alternate costume that's referred to internally as 'Jane' whose 3D model uses a ponytail bone for its hair.
The current speculation is that, with Jane obviously being a female analogue to Jack, Persona 5 R is going to introduce the option to play as a female protagonist much like Persona 3 Portable, and that she'll be included in Smash as an alt costume for Joker.