As an academic, I regularly receive proposals for a PhD study. This is great and I am happy to review applications from smart people who are interested in working with me. Unfortunately, many proposals end up binned because they fail on the following simple criteria.
If you state that you have specifically chosen to approach me because your proposal is relevant to my academic interests, this should indeed be the case. A good way to show this is to read, understand and refer to my writings in your proposal.
It is usually not a good idea to say that you can change your project completely if I am not interested in your current proposal. By saying this, you indicate that you are not excited about your topic. Why would you be excited about any other topic then?
Exception. Some people have methodological skills (or some other exceptional asset) than can provide a good starting point for a PhD study. In such a case, this rule may not apply.
If you are interested in a PhD study just because you want the title, please be honest about this. I rather work with an honest person than with a dishonest opportunist.
Do not take points 1–3 above as a way to cheat your way in. If you are not honest to yourself and open to your potential supervisor, your study is unlikely to be successful.
What makes a good proposal?
A good proposal shows genuine interest in the topic, is based on relevant academic readings, and connects these with the work done by me and my academic group – among other things (I cannot answer the question exhaustively here). Also, note that even if your initial proposal looks promising, it may still take a few iterations before I am happy to consider your full application.
Finally, bear in mind that the proposal is just one of the criteria used for selecting candidates for PhD training.