This is pretty much the kind of template that the NRL should work towards as it saturates every major media market. For instance, with Queensland having 6 teams, that equates to 30 all-Queensland team derby/rivalry games each year - one a week. That game would be the major sporting news in Queensland every week. Particularly if (and they should) the NRL introduce divisions and conferences, which makes those rivalry games matter even more.
With 2 Melbourne teams, you have a game played there effectively every week, boosting the sport's local profile, along with 2 derby games a year. With at least 2 New Zealand teams, you can fill most of the 6pm Friday slot with games hosted in New Zealand and give New Zealand TV a local 8pm prime time game every week. Again - 2 derby games each year.
By 2050 the only other major population centres without a team that are already rugby league inclined would be the Central Coast (450k), a third New Zealand team (Wellington or Christchurch - 500-600k depending on where the 2nd team is), Cairns (350k) and Central Qld (150k-200k depending on the town). You then might consider a second Auckland side potentially, as it will have a population of 2.5 million. Further afield Suva in Fiji might one day become a very long term possibility.
That said, if you can also convince a Sydney NRL team to relocate for their own benefit - looking at you Cronulla... - you can then put a team in one of these areas without going over 24 teams. My preference would be to have 3 New Zealand teams so Auckland, Wellington & Christchurch are all covered, given you 36 games in New Zealand each year and 6 derby/rivalry games.
After this the only other major population centres without a team based locally would be Geelong (400k), Hobart (350k) and Darwin (250k) - all very small populations in AFL areas.