This is a big opportunity for Cristiano Ronaldo and this opportunity has made him the president of FIFA. Ronaldo can play for any club in the Club World Cup, so he will naturally want to choose a club that has good quality star players and a club for which he can do well. Now many may be saying that Cristiano Ronaldo can choose Real Madrid as his team to play in the Club World Cup, but Ronaldo will get the club of his choice, but this is not the case. Now if Cristiano Ronaldo likes Real Madrid and if Real Madrid thinks that they will participate in the Club World Cup with their current team, then Cristiano Ronaldo will not be able to play for Real Madrid. So Ronaldo will have to discuss with multiple European clubs and after discussing, Ronaldo can participate in the Club World Cup for the club that expresses interest in taking him.
I think Ronaldo returning to Real Madrid even for just the club world cup is not possible, Perez won't agree to sign him, I highly doubt. There was a rumor about Sergio Ramos wanting to come help Real Madrid this season when the majority of the defenders were all injure and their defending was even worse, they opt not to go for him even he had no club then.
Also Ronaldo is not the missing piece for Real Madrid to win the club world cup, they still have a well solid attacking prowess rather their main concern should be strengthening their defense and midfied.