If an Perth team is admitted who would they share an rivalry against ?. Obviously if they are with the Bears the rivalry would be against Manly but from an Perth perspective who would be their main rival ?, Melbourne ?.
I say Melbourne because the Storm obviously gained from the Reds demise when they first entered the NRL.
Not all rivalries need to be location based and rivalries don't need to be there from the outset. Some rivalries are developed over time through close contests, spite on the field or redemption in GF rematches.
Yes, a lot of rivalries are location based; but I would more say for most they are derbies rather than rivalries (Eg. Dragons/Sharks, Parra/Penrith). Roosters/Rabbitohs is the obvious location rivalry since RL started... and some would argue that this is a feud rather than just a rivalry.
Look at Storm/Sea Eagles to see an example or a more modern rivalry that is not location based (Battle of Brookie anybody?), and thier back-to-back GF appearances with very different results. Some of the close games between the Roosters/Storm as well as being two of the most successful clubs in NRL-era history has created a bit of a natural rivalry.
I suppose my point is that they don't need to have a rivalry straight off the bat.