There's obviously been a good 10, 20 years between when that person was last successful and now by the way Katy worded it - there's no way she'd invite, say, Madonna and then say in this interview "Remember when she used to be successful? LOL." It'll be someone with huge name/image recognition but who hasn't really done anything recently.
Alanis and Cyndi are good guesses, but I can't see people losing it if they walked out on stage.
Honestly, Mariah is a good shout, but I don't think she'd appreciate being talked about in the past tense, and would
probably still destroy Katy vocally. Birthday has been rumoured as one of the potential songs being included, and Katy has called it Mariah-inspired, if she's required to lip sync then why not.
