It would be very hard to run a European League, because the question would be what happens to the team that finishes bottom of this league.
1 Do they get relegated back to their own top league and that club gets promoted.
2 Who would be in overall charge of the League, UEFA or a newly formed body would do it.
3 Where would we get the refs come from, to run these games, if UEFA say we can't use their Officials to take part in these games.
4 Where would the money come from to pay the clubs to take part, it would need substantial backing of the sponsors to make it happen.
5 Which leagues would take part, and which clubs.
6 What happens to the clubs that take part, and it fails. do they go back to their original leagues they moved from.