Writing a dissertation proposal is not different from writing a standard proposal for a research paper or a grant application. It might be a more complex set of topics that you have to describe within a given frame. (e.g. max. 10000 characters).
The “value proposition” of your proposal is very important: if it sounds reasonable, it will accepted. Otherwise, it will be rejected.
What’s the topic?
Describe the niche you’re focusing on as narrowly as possible.
How will you explore your topic?
Which research methodology will you use and what resources will you need. (time, funding, people)
Why is your topic good for the academic world?
If the expected outcome of your dissertation does not contribute to its field of research, don’t even submit your proposal – keep thinking until you come up with a better one! 🙂