Doubt it's to replace the 717 as they are at most around 15 years old which is basically brand new by Qantas standards. The F100s maybe, but I doubt the ulitisation would justify a newly manufactured aircraft.
The A220 would make sense.... but not until Qantas can pick up a fleet of them that are at least 20 years old of history is anything to go by, so we might be waiting a while 🙄