The thing about expansion is that it's not as simple as just adding a team. Under the current format you need an owner(s) who is firstly willing to stump up the exorbitant licence fee and then is willing and able to fund the team long term, which in reality means losing money year on year. That's why Canberra and Tassie don't exist, nobody is will to put in the cash.
So then, when we talk about expansion using existing clubs (eg: South Melbourne, Marconi, Wollongong, etc), we then have to ask the question, what would they gain from it? At this point in time, I'd argue very little. Running an A-League team is expensive, especially with the grant money heavily reduced. The APL is not going to just gift a licence to any of these clubs, they would need to buy in. Then once they are in the A-League, what do they get? No prize money. ACL spots are nice, but they are a double edged sword as unless you progress far enough, it's a net loss. They would get a bit more broader exposure to grow an audience, but it's still not great. They would get slightly better opportunities to sell players overseas, but again still not great. The only thing they really gain, is the ability to say they are in top tier of Australian football. Maybe at some point in the future it may make more sense, but right now a proper professional 2nd div would be far more attractive to these clubs than the A-League.