Marquee NFL & CFB fan bases just as diehard if not more than European soccer fan bases.
Hell imo there's little in European soccer that compares to southern CFB fan bases.
Idk man. The derby matches in each of the european soccer leagues are super crazy. I dont think any rivalry in american football can rival the derby matches they have over there.
It's a difficult comparison to make. I've been to derby matches, been to Champions League matches, more or less a lifetime NFL season ticket holder as well.
The European leagues and derbies aren't monolithic. Some derbies I've been to had great atmospheres, some didn't live up to my expectations and the common perception generated here in the United States.
My perspective is that in general, NFL and particularly premier CFB venues have far superior game day atmospheres within the stadium. Nothing I've experienced in Europe can match the level of sustained noise that I've experienced at the Oakland Coliseum or Arrowhead in MO. The only exception being a Partizan vs. Red Star derby match I went to in Belgrade, Serbia, no other soccer match I've attended comes close to that.
To be fair there are some key differences between European soccer and American football. Smallest NFL stadium is around 60,000. You can count the number of soccer stadiums in all of Europe that are bigger than that on both hands.
European atmospheres are also more about organized crowd noise and synchronization, where as American football is more about uncontrolled incessant volume.
If you want to hear the quietest, most anxious, least passionate 100,000 fans you've ever heard in your life, attend an FC Barcelona match at the Camp Nou... up until that moment when a crucial goal is scored, then eruption of noise and the release of energy is insane.
Some European fans also incorporate some pretty impressive artistic displays, some also make use of props like flares, smoke bombs, and instruments which certainly add to the atmosphere.
But my main point was that the idea that the NFL needs to tap into European soccer fans to get a taste of real die hard fans, real passion, is absolutely ridiculous. I know that first hand.