I have no idea why people are so against playing UCL knockouts in the Mestalla. A stadium with a much better atmosphere, even than Camp Nou, and with Barca fans that are passionate about the team, as they would have to travel, rather than the majority of apathetic local Socis.
As long as they can assure there won't be a Frankfurt situation, I think it would be legendary.
Would even prefer it to returning to Camp Nou.