Azpilicueta is a tenacious player, he is surely good for Barcelona. But it will be a bit difficult for Barcelona to sign him because of having not enough money. They only want to sign a player who has the free-transfer status. Barcelona also wants to give a cheap salary for their new players, Azpilicueta may disagree with this policy. The only reason that Azpilicueta may consider moving to Barcelona, is once he can get a guarantee to play regularly there. He has a strong competitor in Chelsea, Reece James. Azpilicueta may feel uncomfortable with the situation.
When he began in Chelsea, honestly I was not impressed about him. Gradually he has become a legend in Chelsea with his contribution and even become their captain. It is a very long adventure of Azpi in Stamford Bridge. Even in latest season, he was old and his position is not solid like years ago, any match he plays, he does his best and in general his perfomance is good.
I think at his age, he wants to come back home land, Spain. So Barcelona is good choice for him. He will have many things, a good position to get a spot to World Cup with Spain because Dani Alves left Barcelona already. Back to his home land after many years. Because in Chelsea, he will not be guaranteed for a position in their squad which will negatively affect his position in Spain. Worst, he will miss the World Cup.
It is thing we see this days, players aging and going back to their root to finish their career, It is a good gesture though from the players trying to give back to their parent club and showing appreciation for grooming them at the beginning stages of their career. Azpilicueta going back to Barcelona is a good choice because at Barcelona he will get game time and better exposure in Spain which will help him develop more and stand a better chance of making the World Cup team for the spanish side.