A research proposal maybe 1500-2000 words excluding references. It will include background, research questions, methods (subjects, data collection, tools, data analysis plan). Each section is supported by literature review. So, I would select the topic first and try to address untapped issues by extensive literature review. Untapped issues can be new methods to contribute to the body of knowledge. Or it can be evaluation of an existing or new project.
Suppose you are an engineer in architecture, here you need to think about a new design of a building, this design should has some part new! You explain the benefits of that new part, its cost, and its tolerance to environmental impact, and so on. Regards.
Our Faculty of Business and Management Sciences uses the attached template. I also recent ran a workshop explain to the different sections of the proposal template - this is also attached together with the corresponding YouTube video link - https://youtu.be/A6Gq9s1bPcw! However, proposals will vary across disciplines, universities, faculties and departments, but the basic principles will be similar.