Might need to get rid of the Soccer War, the conflict between the USFA and the ASL. For context:
The ASL teams were mad at the USFA because the National Challenge Cup (nowadays known as the US Open Cup) conflicted with their schedule and financially burdened the teams due to having to travel long distances across the country then return to the East Coast to play league games. The league boycotted the cup and made moves to separate from the USFA, including removing its teams from the National Challenge Cup. Three teams who refused to exit the cup (recent champions Bethlehem Steel among them) were suspended from the league. This caused the ASL to be suspended by the USFA and to be declared an outlaw league by FIFA.
The 3 suspended teams would join a few other semi-pro teams from another organization (the Southern New York Soccer Association) to form the Eastern Professional Soccer League, founded by the USFA as a rival to the ASL. The remaining SNYSNA teams allied with the ASL. Both leagues competed concurrently during the 1928-29 season but the split caused heavy financial strain on the ASL/SNYSNA alliance as their status as an outlaw league meant they had no official support and no foreign players wanted to play there.
The ASL would eventually settle with the USFA and join the EPSL to form the Atlantic Coast League. Two weeks later, the stock market would crash and people would turn their backs to the sport after the whole debacle, which had alienated a lot of fans who saw it as an European-dominated sport and lost interest. After the dissolution of the ASL in 1933, football in the United States was relegated to small, semi-pro mediocrity for decades until the NASL made waves in the mid-to-late 70s.
Having it so that this whole conflict doesn't happen would help the sport survive the Great Depression. Perhaps the USFA could yield, realizing how going into conflict with the budding league would be bad for the growth of the sport and exempt ASL clubs from the Cup, just like how NASL clubs refused to participate in the cup when that league was at its peak.
One thing that could be difficult to assess is what the league would be like post-Great Depression. The format would've had much more time to evolve and the league to expand. Whether or not they would follow American sports traditions like the MLS does or would remain more "European" in style is hard to guess, though if it does want to compete with Europe it might need to go for the later. I still think there'd be conferences, but it'd be like now with an East/West division, perhaps with 16 teams each. Promotion/Relegation would also be needed.
Whether or not it manages to become the #1 sport is also difficult to guess, considering it's not an "American" sport like basketball, gridiron and baseball. To compete with Europe it absolutely has to be at least #2. America has tons of towns and lots of space for pitches so if the sport becomes more popular it'd have a really strong grassroots level. With so many clubs everywhere it'd be considerably cheaper to get into the sport than it is in the country now which would help the growth of it.
A better league means a better national team, and the United States could surpass Mexico and become the frequent North American representative at the World Cup. This would help increase the USA's global football standing massively, especially if they start putting in decent performances.
There's a lot more you could do with this idea, I could keep going but it's a bit late where I live so I'm going to leave it off here.