Been talked about before, which league would the Catalan sides play in. Despite the comment, I dont think UEFA are keen to see teams switch national leagues to that of another country.
Yes, there are examples but I think in the main, they're historic ones of teams having played in a foreign league for 50 years or more. It was UEFA after all that made the Welsh clubs playing in England, give up the right to represent Wales in their club competitions unless they qualified as an 'English' club by winning one of the 3 English competitions (or PL placing).