Expert testimony—or evidence sourced from credible researchers—can help you demonstrate to your reader that this is a topic that the research community is actively interested in. Be careful here! It won't be enough to tell me that your reader agrees with you ("X says that the supernatural in gothic literature is connected to cultural anxieties about disease and death"). Instead, tell me what specific contribution that source has made to the conversation ("X says that the supernatural in gothic literature is connected to cultural anxieties about disease and death because the literature was written and read in environments that were highly toxic, which often led to illness, hallucinations, and paranoid delusions").

In order to really prove that this is a problem worth solving, though, nothing beats a concrete example. Human memory is designed to remember concrete information better than abstract information. Rather than discussing your topic and its potential consequences in the abstract, give your reader a real-world example of those consequences (positive or negative). For example: 

Take “scarcity” as an example of an abstract idea. Scarcity can be explained as follows: the rarer something is, the higher its value will be. But this description contains a lot of vague terms, such as “rarer” and “value”. How can we make this idea more concrete? We could use a specific example to illustrate the idea.

Think about an airline company. If you were to try and book a flight four months in advance, the ticket prices would probably be pretty reasonable. But as it gets closer to the date of travel, there will be fewer seats left on the plane (the seats are more rare). This scarcity drives up the cost (value) of the tickets. This is a concrete example of scarcity, which is an abstract idea.

You'll conclude this portion of your proposal with a statement of purpose. Most purpose statements begin with "The purpose of this study is..." The purpose statement is a line or two that describes exactly what your literature review can contribute to the conversation. Once again, make sure you're not making any promises you can't fulfill - we won't be fundamentally changing the world with this project. Also, remember your reader— professionals in your field who are probably about to begin their own research projects. What can your literature review do for them?