I know some people have trashed soccer saying, "They've all been saying soccer for 10-20 yrs and it hasn't happened yet.....blah, blah, blah....." It just takes a lot longer than that. But, I think it's going to be soccer.
1. Soccer is one of the largest and widely played youth sports.....and continues to grow rapidly. Basketball is also played at a young age and is very popular.
2. MLS - Major league soccer has only been around for a short while now, but continues to grow rapidly....and the product continues to improve.
3. Change in population.
Think about the data above. Most Asian countries list soccer/basketball as their #1/2 sports......followed by cricket, table tennis, etc. Most Latino countries have Soccer as #1 by far. Also, most Black countries list soccer as their #1 sport by far. Now, certainly a good portion of this growth is due to breeding within the US population, but it's also growing due to immigration.
4. Change in other sports. As much as I love football, there are major rule changes very frequently. The current sport is quite a bit different than it was 30 yrs ago......and much different than it was 50 yrs ago. While the intention of these rules are often pure, they also "might" take away from the fan or player enjoyment.
Now, contrast this to soccer. Since this is mostly a world sport, almost all countries use the same rules/format. This inherently makes it harder to make significant changes. And outside of adding technology (replay, goal-line tech, etc), the rules haven't changed a great deal in the last 30 yrs. This leads to a very consistent product.....at least as far as rules go.
5. Multi-level - Let's take football for instance. Living here in Kentucky......you pretty much just have the HS, NFL and college. But with soccer, you have HS (both men's and women's) college (both men and women's teams), MLS, USL, NWSL, USWNT, USMNT, etc.
6. World Cup and other large tournaments - In the current American sports, you always have the regular season followed by playoffs and a championship.....then it's done for the year. With Soccer, you have the regular seasons.....but also have rotating tournaments like the recent Gold Cup......or of course The World Cup.