This, this, 1000 times this.
Ball golf's "grand slam" evolved into what it is today. The original "grand slam" was invented by Bobby Jones just because it represented the four titles he wanted to try to earn all in one year: US Open, US Amateur, British Open, British Amateur. Then it became about the pros only, so the two amateur tournaments dropped out and the Masters (the only one held in the same place every year) was added. Then once the PGA Tour was created, the PGA Championship became the fourth.
We're probably still at the stage pre-PGA Tour where there are two clear cut "majors" (Worlds, USDGC) and then a whole bunch that could make the cut to be the third and fourth eventually.
Frankly, I think one of the criteria for being a "major" in Grand Slam ball golf style is that one must earn their way into the event. It can't be a tournament you can just hang out online at the right time and get yourself into. USDGC fits the bill as requiring players to qualify at a satellite event (or the day before). Open Worlds had a points requirement that could be easily brought back, so that fits too. Maybe the DGPT Championships fits as a third at this point since you have to earn DGPT points to get in. No other tournament on the DGPT or NT has minimum criteria. If there's an open spot, anyone can fill it.