Pretty tough questions!
Any more teams spreads the talent pool out further, which is a problem, but fixtures are already such a mess with each team playing others sometimes once, sometimes twice.
The only way to properly fix the fixtures without drastically increasing/decreasing season length is conferences. Would allow 18 teams pretty easily, or 16. (ie play everyone in your conference twice and other conference once = 25 or 22 reg season games).
To keep the talent pool though you need to either relocate or cut & replace a team. Eventually the new markets may help increase talent pool ongoing.
A 16 team conference could have two NSW teams cut for 2 of Perth/Brisbane/New Zealand, or conferences could be more randomised