医者

In this sentence, "将来" (shōrai) means "future," "医者" (isha) means "doctor," and "なりたい" (naritai) means "want to become." The phrase "と思っています" indicates a desire. The speaker expresses a wish to become a doctor in the future. So, what they want to become is a "医者" (doctor).

「将来」は「future」、「医者」は「doctor」、「なりたい」は「want to become」です。将来なりたいものは「医者」だと明確に述べられています。