If you asked a consultant (say from America) to review the NBL and suggest strategies for expansion, they'd do a BIT of research, look at AFL, read a bit of local histories, and recommend "new" teams in Western Sydney, South-East Melbourne, and Fremantle.
Is their a natural and historical rivalry between Freo and Perth? Yes there is.
Unlike most capitals, Perth is not a port. Freo has always been a separate city, and the two have grown into each other. That it is now effectively just another municipality within the Perth Metro area, does not detract from its history or self-perception.
Is that enough to base a 2nd team on? In the circumstances that prevail, no I think not.
Basketball is a relatively modern sport, and relatively minor. It is not entrenched in the local culture like footy, or even soccer.
From the day the Eagles entered the VFL, prominent identities began agitating for a Freo team. Their angst was heightened by the fact that many of the Eagles greats were in fact "Freo boys." When the new Freo AFL licence was finally announced, the Eagles were still a relatively young club.
By contrast any new NBL team needs to fight 30+ years of entrenched Wildcats support & success, without any natural counter-culture to build on.
The Dockers have thrived because footy in WA is BIG business. Even when they sucked (which was often) they made money. They have also benefited because for some time now, if you wanted to be an Eagles member or Sponsor, you needed to go on a waiting list.
They will never attract the same level of support that the Eagles do, but with the money in AFL, they don't need to.
By contrast, the bare minimum another NBL team would need is their own personal squillionaire.
The AFL was more than happy to see Freo enter the league, because all the Melbourne clubs were bitching about the Eagles being a "State Team." (That must sound familiar to Breakers' fans.) Regardless of the draft that was introduced specifically to enable said Melbourne clubs to pillage the local talent.
Even now, the sponsorship and membership money available in Perth gives the local teams a huge advantage being spread over only two clubs.
In the NBL, nobody cares about local talent, and there isn't really enough sponsorship money to go around.