If Ronaldo want to play in Spanish league, he will only prefer La Liga Santander which is where the club (Real Madrid) that he was before he went to Juventus, then back to Manchester United. With how he is in Manchester United now, do you think clubs like Real Madrid, Barcelona can prefer C. Ronaldo to play with them again? He is getting old in football and that may not be possible. The best is for him to later join MLS is what I can think, but I do not think it is yet good time he should join.
I don't think yet with team like Real Madrid and Juventus want to sign in again due Ronaldo age have been 37 years old and actually he doesn't at the top performance right now trough when he still under 30 years old. Realistic for Ronaldo joined wit MLS league before retired from football or he could try playing at small team but doubt want to pay expensive salary from Ronaldo. Probably Ronaldo have bigger ambition keep trying to play on higher level club exactly have team participants on Champion League.
Can't stoppable with Ronaldo ambition actually he want play at European team and ever rejected most fantastic offer from Asian team because he want end his career keep playing at Champion League.
Ronaldo's age is a big factor that is affecting his performance, there is a say that 'You can't cheat nature'. I don't think any big club in Spain will love him play in their club due to his present performance in Manchester United. He has achieved so much in football but his age is putting a stop to that.
If Ronaldo wants to continue playing football,he should leave England since England football is strong and needs someone young that can coupe with the football style. He can go to any club of his choice but not his former clubs because he is not fit in anymore. Maybe clubs in America can will always appreciate his presence as their player.
It is good Ronaldo quits football as a super star now that he is fading away to keep his good records. Instead of quiting when he has already faded that can down grade his personality a bit.