In a research proposal we have to write a theoretical framework. In this section should we always refer to stablished theory? If so, then is it not limiting the creativity of a researcher?
One way you can think of the theoretical framework is as a way for you to explain your understanding of the world, to your audience. By using previous theories (and not always developing a new theory, which could come later), you also situate yourself in the world of others, showing that you understanding their work, acknowledge their hard work, and show that you fit within it. There are of course ways to develop theory, such as in research using grounded theory, but that can come later as likely your committee will want you to still rest your work upon previously established theories to begin with. Good luck!