First, it should be noted that the only way to really discover new ideas for an original thesis is by mastering the relevant historiography and beginning archival work. Your MA advisor should be able to point you toward the relevant literature. That said, some possibilities might be in the area of the rhetoric of the Truman Doctrine. How did Truman and other government officials articulate the responsibilities the United States was assuming with this approach to foreign policy, both at home and abroad? Obviously, they were acting out of fear of communism, but how was this fear expressed? Another might be to look at the actual effects of the doctrine. Another might be to look at how people in the United States and abroad responded to it, looking at how dissent from, and support for the policy varied. For a smaller project like a MA thesis, you might do a rather local study, consulting newspaper editorials to gauge public opinion in different regions. Again, there are many options, but the only way to really do original research is to master the literature. Then you have to get into the archives.