I'm not sure thst geography will provide a differential because broncos formed as an all of brisbane team and didn't grow geographically from a suburb. They are pretty equally supported and opposed in all areas of brisbane..there is no north v south v east v west rivallry with broncos. I don't think geography will do it unless they are genuinely in seq but not brisbane. Alternately they need to be the brisbane somethings that are counter broncos. I also wonder what the support base will actually look like considering most anti broncs in brisbane probably already support an nrl team. 2nd team def good and needed in bris but who will actually support them. ..maybe that's the clue in how to differentiate a new team.
Most rivalries aren't built on geography anyway, they are built on ideology and circumstantial differences that define separate groups.
For example the rivalry between east and west Sydney isn't actually about the geography, the geography is just a visceral representation of the class divide as it's more expensive to live in the east than in the west so naturally as a result of that more "poor people" live in the west and more "rich people" in the east, it's a class based rivalry.
Though what differences would be the best to exploit in Brisbanes' case is probably better answered by someone from Brisbane rather than me, similar differences that could be exploited to create a rivalry definitely exist in Brisbane, they exist in every population to some extent.