Nowhere is going to crack 40k from day one, and probably not even the 20k average Freo got in the first few years. Medium and longer term WA3 and even WA4 are far more likely to succeed than Tas, Canberra or SA3.
In the shorter term, yes, the interest will be small. There will still be more scope to pull in the vital secondary and tertiary sponsors that keep clubs running. Any extra team(s) will always stay smaller than WC or Freo, but still become mid-sized in 20 years or so. growth will far quicker than GC or GWS.
And for WA fans, it means more games without travel if there are more away games in the home city. (Although not if a promotion-relegation system was brought in, unlikely but makes sense to me at 24 or maybe 22 teams.)
For broadcasters it means extra games in WA, which means more prime time in the east. And there may be some WC fans sick of waiting lists who jump on WA3 quickly, whether jumping ship or simply to get access to AFL games. Some of the latter may well become attached to the team they get to see.
Joodalup-Wanneroo or whatever would need to have its own ground for smaller home games, preferably one with scope for significant expansion later on.
Perth City would perhaps be best that way for a while, but without the expansion needed at their venue as once they grow they can just move into the 60k one. There isn't a geographical distinction.
However, if going to four WA teams both new teams need to be introduced at once. That appears extraordinarily unlikely.
