FAN LOGISTICS
If the competition is only between these teams, and they will thus leave their national leagues, then how would fans travel for games?
Real Madrid vs.Barca, or Man United vs. Liverpool, already are trips many fans of these stated clubs are used to. Man United vs. Bayern, given their respective long tenure in European competition, is another regularity. But then every other week, for away games?

To travel within a country often can be expensive. But between countries would heighten expenses greatly.
Visa issues will also be a factor, and especially for English fans given Brexit and a possible change in free access to the EU. Policing is also another issue, as there would need to be more international co-operation on hooligan travel.
Clubs could readily absorb this - they already have chartered/hired planes, trains, and coaches. They also can afford first-class accomodation, tailored to training and their dietary and fitness needs. Fans are different though, and their needs are being ignored here.
It remains to be seen if the cost of fan travel will be subsidised or not.