A stand alone literature review has a different purpose to a literature review that is part of a research study. When a literature review is part of a research study its purpose is to justify the research question that the study addresses. It needs to show readers what is already known, where the gap being addressed by the study is, and why the gap is important to fill.
In contrast, the purpose of a stand alone literature review is to answer a question itself, based on analysis, evaluation, and synthesis of published literature. The question may be a general one like “what is currently known about..…” or “what does current evidence suggest is the best strategy for…..” but it can also be a more specific one such as “what factors are associated with the development of…..” or “which therapy for (condition) has the best long term prognosis…”. The question being answered needs to be an important one to answer and you will need to explain to readers why it is important in the introduction to the literature review.
When planning your literature review, you may start with a general topic, but its important to work on defining a research question early on. In order to define your question you will need to do a lot of reading, and also perhaps create summaries of what you have read, but you will not really have started your review until you know what the question is that your review will answer. The research question provides the focus for your review by defining the topic more specifically and ensuring it is feasible. For more information about formulating a research question, see this post from Sheffield Hallam University.