In an exclusive interview with Portuguese broadcaster TVI, the Portugal international revealed that Spain is really in his mind.
Despite his dream, Ronaldo admitted he would stay in England for "10 or 20 more years," adding: "I feel well in England."
The 23-year-old said: "Many players are talented, but if that talent is not developed they will never go anywhere.
"I keep improving everything I have. Four years ago I was not the player I am now. I improved and I better understand the team-game concept.
"I think I still can improve. My learning process has not finished yet."
Ronaldo has scored 27 goals with United this season and he wants more - but admitted that there is a limit.
He explained: "Last season I scored 23 goals and some people doubted that I could do it again.
"This season I have just scored more, and things are going fast.
"I aim to score more and shine even more, helping the national team and my club - but to score 50 goals is a bit hard."