User blog comment:Yorkieman95/Is it OK if I write reviews about fanfics?/@comment-104549-20150126034817

You wouldn't be the first; I know Atarumaster88 has reviewed some fan-fiction works, as you can see on his User Page. In a sense, the entire Featured Article/Good Article/Featured Work process is a way of collectively reviewing others' works. I do recommend that you post your reviews on your User Page or a sub-page you create just for that purpose; people might be annoyed if you post lengthy reviews on the Talk Page or Comments section of an article or fanfic itself.

Out of curiosity, what would inspire you to start doing reviews? Atarumaster88, for example, is an Archivist, so in a sense his reviews are part of his "job".