He has already proclaimed via social media that the whodunit genre is very malleable so don’t be surprised if Johnson sets his murder mystery during a musical production.
Andrew Scott said in multiple interviews that he'd want to star in a musical (but lacks confidence), so a film that allows multiple takes and editing could be the perfect place.
This theory for the third installment of the “Knives Out” franchise might be outside the realm of possibility, but with so much musical talent in the cast, you never know.