With the expansion of gambling the NFL does have reason to expand because it would mean a larger inventory of games to bet on. The regular season can't go beyond 18 games.
Also, there is always the potential for more TV contracts. When you had 3 national networks you were basically locked into an AFC package, an NFC package, and a Monday night package. Then ESPN comes along and you add Sunday night. The NFL Network gets created and you add Thursday night. An All-Europe division will get you a Sunday morning slot to sell. There was talk before the last deal of scrapping the AFC/NFC model as well. That's all before you start thinking about Tuesday and Wednesday night games. I hate having my team's games on Thursday (there is always someone who isn't fully healed and the quality of play sucks) but it makes money so the league sticks with it.
Does the league directly benefit from the gambling? Are they getting a cut?
The Inventory for TV isn't a significant add for the NFL like it is in other sports. The AFC package, the NFC package, MNF, SNF and TNF... the networks are paying for EXCLUSIVITY: one exclusive time slot per per week where they have the only NFL game on at that time. (And the AFC/NFC Sunday afternoon one has one shared time slot). Adding more teams doesn't create additional time slots, it just creates more choice for games to be aired in those time slots.
The NHL is talking about expanding to 34 immediately after expanding to 32, doesn't compare to leagues expanding to 32 from 30 imo. If baseball jumps into Portland and Montreal and then almost immediately starts talking about how they're going to also expand to Austin and Nashville, then we can talk comparisons to where the NHL is
Well, the NHL ISN'T talking about 34 teams at all. It's been Buccigross tweeting about Atlanta and therefore Houston, and Salt Lake talking about how they want a team. (And of course, all of us!). The NHL is doing their usual "Not looking to expand at this time." (Which is what they always say until it's happening).
And the major difference between the NHL and EVERYONE ELSE is that MLB, NBA and NFL can't put teams into Ottawa, Edmonton, Calgary and Winnipeg. The NHL has Canada and Canada is awesome. While NBA, MLB and NFL can work in Vancouver and Montreal, the NHL is always going to be able to have +4 minimum over the other leagues because of the "small-market" Canadian teams that support hockey.
I've also long suspected the endgame for MLS is a situation where they have a closed pro/rel league with MLS 1 and MLS 2 so it makes sense for them to keep expanding until they hit 40-ish teams
If that's their endgame, they're screwing it up royally. They should have started MLS-2 first when they had like 16-18 teams, then sold 40 expansion teams into that league; and promoted teams to MLS with no relegation until the two leagues were balanced. Because no one who paid into MLS is going to want to risk being relegated to MLS-2. That's why MLS chose a closed system to begin with.