Personally I don't see a magic left-field option appearing. I would suspect Noah Lolesio, Bernard Foley, Tane Edmed, Ben Donaldson & Quade Cooper (dependent on recovery) are our options.
Jame O'Connor is a possibility if he has a blinder of a season and Hodge is a no.
The other options of Lawson Creighton, Gordon Carter, Tom Lynagh, Reesjan Pasitoa etc.. are all future prospects.
Bryce Hegarty and Jack Debreczeni are the left-field options, but I just don't see it based on their past showings and total lack of international experience. They'd have to have an exceptional Super Rugby season. I do think Hegarty is somewhat under-rated at Super Rugby level though. Talk of Hamish Stewart isn't going to happen in the short-term. He need's time to prove himself in the fly-half role first having only had limited stints at the Reds (less than 10 times I believe).
The rate the fly-half conversation is going, people will be proposing players like Mike Harris, Andew Deegan or Mack Mason soon... or seeing if Matt Giteau or Berrick Barnes can be brought back on life support.
I have no doubt that the coaches have a list of every possible current option from Jonno Lance upwards. My gut says that Cooper may struggle to return in time (and they have concerns that he won't) such that we may end up seeing a bit of Bernard Foley in this RWC.
I'd be curious to see someone like Isaac Lucas retutrn to SuperRugby (or even shift to a higher standard competition) to see if he's got what it takes.