It's an interesting flow-on effect of not having promotion/relegation. After a really poor season that results in relegation, at least the club is more than likely to have an excellent following season. Keeps the fans somewhat healthy!
Yeah expansion to 20 teams and pro/rel with a 10 A1 & A2 divisions is the answer.
Within that come the end of season you's have clubs fighting to get into playoffs, win titles/promotion or avod the drop. Basically ever club with something to play for except likely the bottom 2-3 sides in A2.
You'd probaly have the big 5 spenders (AFC, SFC, WSW, VUC & Melb City) always in the A1 division. Plus hopefully the Nix most seasons. Clubs like Roar and Jets you could see yo-yoing a bit between.
Start season with clubs from both divisions in some sort of crossover League Cup type comp with pools that goes for say 6 weeks, then 18 rounds H&A regular season A1 & A2, then playoffs
Of course all a pipe dream at the moment with some clubs struggling financially, and the mooted 14th new team Canberra still yet to happen.
But maybe the new NSD is a success and from that a few clubs with solid crowds, and sound financials can go up to the A League.
Maybe rather than 20 teams, go for 16 or 18 but still have the 2 division split. Certainly pro/rel would add some interest, especially given would be the first professional sports league of any type in Australasia with pro/rel.
Whether this could ever happen you feel depends alot on how the new NSD goes. Crowds, sponsorship etc etc