The major stumbling block is that the big European clubs don't really want a European League outside of the Champions League.
United, Madrid, Barca, Munich etc, are all happy being top dogs in their own country. Its more lucrative winning/challenging for domestic trophies every year and playing in the Champions League than it would be if they all only played in one European League in which only one of them can win.
What they all want is to make sure the governing bodies always know that them breaking away is an option, so they can use that as leverage to gain more influence, and therefore more money.
If you look at United, since the PL invented football they have won the league at a better rate than every other year. TV revenues increase with every deal, and they get the biggest share of that. And they are in the Champions League every year, so rake in the cash from that. Why would they want that to end?