Ronaldo can play at any club, but Ronaldo will not join just any club. Only a few clubs are willing to pay his high salary at his age, the rest will not be able to come close to him. I also don't think Ronaldo will easily return to Europe and get a place at the club he wants, but you never know what deals will happen in this sports industry.
Signing Ronaldo at the age of 40 is a business deal that some rich clubs can still consider. Many people are interested and fans of Ronaldo so the club's business will certainly make a lot of money. Instead of signing him to win titles, I think any club that would sign him would just be thinking about how to make money from him.
No single club can actually sign Ronaldo for such a huge wage, personally I think if Ronaldo leaves Al Nassr only clubs in other countries like Asian countries can actually afford his wages, no single club in Europe can get him for such a huge salary even if it is for sporting reasons. The only place again will be the major league soccer, the league is looking for recognition and having both Ronaldo and Messi will be a big impact.
Personally I don’t see any reason why Ronaldo should be signed by any club looking to compete for top flight tittles in top European competitions. He can actually play in Portuguese league with Sporting just to help them raise money but he has ruled that out. Any club signing him will have their younger players affected except he accepts to play as second option from the bench which is seriously not looking to happen