By DAVID E. SANGER President Donald Trump on Thursday said he should have a role in choosing Iran’s new leader, and that Mojtaba Khamenei, a son of the former leader Ayatollah Ali Khamenei who appeared to be the leading candidate to succeed his father, was an “unacceptable” choice. Trump’s comments, in interviews with Reuters and Axios, were the most explicit he has been yet about his vision of an American role in creating a new government in Tehran. They made clear that Trump’s model for...