In many countries an application for a PhD position includes a written research proposal, so my questions is what are some advises/strategies to come up with a good topic/idea for a PhD research proposal and how can one assess the quality/fruitfulness of an idea? As an undergraduate student one just doesn't have the experience to foresee which ideas might have promising research results and which probably won't have. (And I doubt that potential supervisors have the time to comment on every idea of every potential applicant in cases where it is possible to establish some kind of contact before the actual application.)
There are already very good answers above; I would only like to add that you should also take into account whether you can get a scholarship/funding for your research topic. Your personal interests may not necessarily align with those of your potential funders. Consider how much you would be willing to compromise your own interests to be able to receive a scholarship. Most people cannot support themselves financially through the course of a PhD programme, so this point is not to be underestimated.
If you apply for a government scholarship for example, they will likely want you to study a topic that is of high policy relevance to them and you need to think about whether you can offer that or modify your original ideas in such a way that they will meet the policy priorities of the government at the point of application. Governments tend to publish their strategic priorities in various documents online, so it will not be difficult to make the connections between your research and their needs.
University departments giving scholarships tend to be more flexible with regard to research topics as long as the quality and originality of the proposed research is high, but again, it would be best to get an opinion from a member of the department on the chances of your proposal attracting sufficient interest that it will get funding.
Without funding, it will be close to impossible to do a PhD.