Yeah, I suspect that the call will likely either be "open slather" OR "Everywhere except for SE-Queensland".
Open slather sounds good, but the chances are that some of the losing bids from the "Brisbane 2" contest will just re-emerge.
I suppose it means the NRL gets a refreshed look at the Brisbane bid scene (eg what brand Brisbane Easts choose this time, how a Western Corridor bid positions itself, what other SE-Qld bids pop up that weren't there last time).. but by not explicitly ruling out that area, it risks the ire of the existing NRL clubs there, not to mention News Ltd with their Broncos stake - and that might be too many headaches to raise right now.
My hope is that the NRL open it up to everywhere EXCEPT SE-Queensland - fish for bids that are in a new, COMPLETELY unrepresented places. They already have a read on who's interested around the Brisbane area (or at least, who was putting their hands up 2 years ago).. and that market can be revisited later, so let's see what else is out there?
As you say, it's a good way to get a guage on not just the 18th team, but also options for 19th and/or 20th teams.
It's very possible that an unsuccessful "18th team" bid from outside Brisbane will get the nod as team 19 or 20, after being asked to brush-up their business case a little - and likewise, one of the unsuccessful bids for team 17 may have their time in that expansion phase as well.
What if only decent bid is in Brisbane (which is probably the case)?!