First let me say that nothing I write here is meant to degrade American Football, only to give a European view on it after several of you promoted it here. I don´t think that American football has any chance to ever gain more than a niche in Europe for several reasons. The most important is what I´d like to call the "culture of sports":
Europeans usually demand a team sport to be as uninterupted as possible, "to have a flowing grace" as a friend of mine once put it. This is the case in football or less popular sports like team handball or field hockey. Ice hockey or basketball can have this flow to a certain extent as well, which is why they are pretty popular in parts of Europe. I can imagine a European tolerance for the constant interruptions in baseball, because it is radically different, but AF has enough similiarities with established sports, that spectators will be pushed away by the interruptions.
Connected with this is the reputation of AF in Europe as chess on grass. This is a derogative term used for highly tactical games where players "kept close to the text books" and rarely showed initiative. In football by definition this is a boring game. AF now relies much more on tactical guidelines and the brilliant improvisations which excite European spectators are unlikely.
Another problem of AF are potential recruits. Indeed no one will go to a game to watch only second rate American players. But in America sports are closely interwined with the educational system. Here that is not the case. You usually get no support for playing a sport and no advantages like college stipends for it, not even in a popular sport like football. Instead you have mostly to pay for membership in a club and for your equipment, something which is not cheap for any sport, much less AF. Getting rid of some protections in youth leagues might make it easier and also might reduce the reputation of AF players as wimps which can´t take a hit unprotected.
There are additional smaller things but they only will underscore that what makes AF so popular to spectators in the US (short spikes of action seperated by breaks) is exactly what it makes so unpopular to the Europeans. And I can see no way how to change it without changing the sport entirely.