I still like more the idea of sending him to smaller club next season. For me he's still 2 years removed from being good developed and experienced enough to be part of Barça's squad and being a starter in top 5/6 La Liga club will be next step in his career, possibly starting in 2017.
Yes, it would be great if he could get into starting XI in club like Villarreal (as Denis did, but don't forget he's 2,5 years older and was also playing solid in Sevilla last season before Emery decided to not trust him anymore) and also playing in European competitions already next season but there is no guarantee for that. I think next season it's crucial for him to play as much as possible and it's better to play 3.000 minutes in a club like Rayo than 1.500 in Sevilla.