Only way I can see this work is if the 30 existing teams split into 2 divisions of 15 (or 16 in case of expansion). It makes no sense to involve the G-League.
I think the league would have to expand, but that would be interesting. An upper tier of 24 teams and a lower tier of 24 teams would be a way to make it work. Add like 10 teams in Europe and the rest in North America. It would be a way to have the expansion the league wants, but with some caveats. All expansion teams are in the lower tier and the worst 6 teams go to it for that first season. With the significant increase in markets, you could probably make the money work to where it might actually work. 1st tier teams are the first 24 picks in each round of draft, 2nd tier are the 2nd 24 picks in the draft, so you just expand the draft to all 48 teams, which will also allow for trades between teams in the different tiers.
So add something like
"European" Teams - London, Paris, Madrid or Barcelona, Lisbon, Athens, Istanbul, Berlin, Rome, Tel Aviv (I'd say Moscow, but kind of hard to include them at moment, so I will leave that 10th spot blank)
N. American Teams - Seattle, Las Vegas, Kansas City, St. Louis, Louisville, San Diego, Buffalo, Nashville, Albuquerque, Tampa, Baltimore, Montreal, Vancouver, Mexico City, Guadalajara, El Paso/Juarez,
Something like that.