NHL is a bad example - Canada provides twice as many players as America does. Popularity of hockey in Canada is much more than in the USA. The Aus-NZ comparison would be rugby union where a hypothetical 12 team trans-tasman would probably be 7 NZ and 5 Aus.
In the other leagues Toronto is the only Canadian city with a team. Toronto is also a metropolitan area of over 6 million people and is only smaller than five US metropolitan areas - New York, Los Angeles, Chicago, Dallas and Houston.
NA sporting leagues are also ran completely different. They are private organisations ran solely for profit with teams owned by businessman and often relocating for profits. Each of the major NA leagues is also the highest level of that sport.
On the other hand the BBL is run entirely by CA which owns (through the state cricket association) every singel franchise. The BBL is also meant to be a level below international cricket as shown by CA not freeing the likes of Smith, Warner and Starc to play BBL.
Shaun Marsh is Perth's best batsman and he was pulled from the BBL final in order to go to New Zealand early to prepare for a 3 match ODI series that CA considered so irrelevant that neither Smith or Warner played. That wouldn't have happened if CA meant for BBL to be the top level of cricket in Australia.
CA gets all the revenue from BBL.
The aims of the BBL are as follows - 1. get more kids interested into cricket again (especially those from outside of the traditional anglo-celtic background) 2. provide revenue and profile for domestic cricketers to make cricket more attractive than AFL as a career. Most good athletes in VIC, SA and WA play both AFL and cricket as kids and in the end more choose to play in the AFL than play cricket. Mitch Marsh and Will Sutherland (son of James Sutherland) the notable exceptions.
Revenue is also important but given CA was willing to accept a much smaller offer from Channel 10 last tv rights deal in order to get BBL on free to air tv every day its clearly not the main priority.
Now the last BBL team had 8 rounds with each team playing the other seven teams and then having an extra match against its "rival". This means that the logical extension for the BBL is just to increase the number of rounds until it is a 14 round home and way system. That would be the easiest way to increase the content.
But lets say what if BBL wants to expand by bringing in more clubs.
Its got to be noted that the only thing New Zealand can offer is revenue* and expanding the player pool. Now revenue is also iffy because CA would have to share any revenue from NZ sides with New Zealand (otherwise why would New Zealand agree). Not only is cricket a much smaller sport in NZ than in Australia but the population is much smaller and while Australian fans love Aus v NZ rivalry they don't care about Australian cities v NZ cities let alone NZ cities v NZ cities. A match between two Australian sides will always get more neutral Australian fans watching. State v State rivalry is the big thing in Australia (with the capital city often being equated with the state eg Perth v Adelaide is Western Australia v South Australia).
New Zealand has a population of 4.8 million people. The population of Sydney is over 5 million and the population of Melbourne is 4.7 million (and will overtake both New Zealand and Sydney soon). The population of Queensland is 4.9 million people.
So lets look at the possible franchise locations in Aus/NZ
Code:
# City State/Country Population
1. Sydney New South Wales 5,029,768
2. Melbourne Victoria 4,725,316
3. Brisbane Queensland 2,360,241
4. Perth Western Australia 2,022,044
5. Auckland New Zealand 1,534,700
6. Adelaide South Australia 1,324,279
7. Gold Coast Queensland 646,983
8. Newcastle New South Wales 436,171
9. Canberra ACT 435,019
10. Wellington New Zealand 412,500
11. Christchurch New Zealand 396,700
12. Hamilton New Zealand 235,900
13. Sunshine Coast Queensland 317,404
14. Wollongong New South Wales 295,669
15. Hobart Tasmania 224,462
16. Geelong Victoria 192,393
17. Townsville Queensland 178,864
18. Cairns Queensland 150,041
19. Darwin Northern Territory 145,916
20. Tauranga New Zealand 137,900
21. Napier New Zealand 133,000
22. Dunedin New Zealand 120,200
We'll ignore the fact that logically Melbourne and Sydney could support 3-4 teams each and Perth and probably Brisbane and Adelaide could support 2. In any case I am 90%+ convinced if CA were to bring in new franchises they would be from new cities.
This list has every city in Aus/NZ that has a population than Dunedin. I picked Dunedin because due to cultural and historical reasons it would likely be the hypothetical fourth NZ franchise. Given that its got a very low population and there are no less than 15 teams on that list with larger populations that don't have a team it would never be considered for a BBL franchise. Hamilton would be the other option and its larger than Hobart which does have a franchise but Hamilton would never be considered either. The only reason Hobart has a BBL team is because Tasmania have a Sheffield Shield side and every Sheffield Shield team got a BBL franchise.
Realistically speaking we can rule out almost every city smaller than Christchurch from being considered for a franchise (there are two exceptions to this in Geelong and Townsville but I'd go into that later).
My belief is that within a decade Canberra and the Gold Coast will have BBL teams. Both are hosting BBL matches this year. Gold Coast, Canberra and Newcastle are quite sizable. Gold Coast has the benefit of setting up a South Queensland rivalry with Brisbane and South Queensland region has a population of 3.4 million people. The Gold Coast is hosting the 2018 Commonwealth Games and due to the AFL already has a 25,000 seater oval stadium that is suitable for hosting cricket.
The Canberra stadium is smaller but has already hosted ODIs and BBL finals and would get government funding to improve the stadium if it got a franchise already.
Newcastle has the population (Hunter Valley has well over 600,000 people) and a strong regional identity and would be a good candidate but unfortunately due to not being an AFL town it doesn't have a suitable stadium so would be behind Canberra and Gold Coast.
Now Auckland is the elephant in the room. Honestly speaking if NZ were satisfied with having only one BBL franchise then Auckland would probably get a team ASAP. However having teams in the BBL would forever relegate any domestic NZ competition to second rate so realistically NZ would be foolish to agree to enter only one team. NZ can only accept three teams - Auckland, Christchurch and Wellington otherwise it won't have an acceptable national footprint.
But unfortunately Christchurch and Wellington offer near nothing to Australia.
I think eventually (eg 20+ years down the track) there will be Newcastle and a second Perth team and that will be it.
Actually Geelong and Townsville would also be ahead of Christchurch and Wellington. Both are smaller in population but have a proven track record in supporting professional sports leagues (The Geelong Cats have twice the average attendance of any NZ super rugby side).
If they were to want to go past 12 teams that would be it but I can't see why they'd do that because at that point franchise cricket has become the main thing.