One prevalent issue is the interplay between poverty and illiteracy. If you are in the United States, you will surely be able to find many examples of the relationships between family income and literacy achievement, as well as articles debating the reasons for it or proposing possible solutions. Worldwide, poverty and literacy is also a problem, and in some developing countries it is particularly dire. If you tackle the poverty and literacy issue, you may need to focus in on a sub-issue. For example, you could write on a group of sources that that explain why poverty affects literacy, or you could synthesize a group of arguments proposing solutions.
Another issue with local and global implications is the problem of literacy instruction for marginalized minorities, especially those who speak a language other than the dominant language in the area where they live. Many literacy experts agree that it is best to teach children to read in their native language, but that is not always possible in practice. In the United States, for example, public school instruction in Spanish for native Spanish-speakers tends to create controversy. In South Africa, to take another example, there are eleven official languages, but many parents who speak African languages at home elect to send their children to school in English. Again, this is a large issue with many implications, so you would probably need to choose a narrower sub-topic for the focus of your paper.
Below are a couple of links to websites about literacy issues and synthesis essays.