I'm hardly the first person to bang the drum for more diversity in literature, and in recent years there has been wider recognition of the value of hearing different stories from different types of people with different perspectives. Some people may still miss the point and kvetch about 'heavy-handed message fic' taking over genres like science fiction and fantasy, but with thankfully little credibility.
Perhaps this is why I feel disappointed when an otherwise good story is populated with a cast of cisgendered, heterosexual, able-bodied, white, male-identified characters. Sure, it may have a couple of characters who don't fit that profile as love interests or to help or hinder the white guys in their adventures, but ultimately the stories are not theirs. We've all heard this complaint before, and as readers the only way we can encourage diversity in literature is with our wallets.
As an editor, however—and especially as an editor who works with independent authors—it's my job to help shape the story (and not just in a postmodern 'Death of the Author' kind of way) before it reaches the general public. The question then becomes one of remit: I am the editor of this story, not the author. Ultimately the author has the final say over the content of their text, and a story about a white guy may be the story that they want to tell. Of course, anyone wishing to write that kind of story—and anyone wishing to read it—is perfectly welcome to do so. To mangle a phrase: some of my favourite literary characters are white guys. My job is not to tell authors that they can't, or shouldn't, write about white men.
My job is to make each text as clear and appealing to its intended audience as possible. And if I'm doing developmental or structural editing on a work of fiction then that job can include tactfully querying whether it's vital to the story that all the cis, het, able-bodied white dudes possess all of those characteristics. I might mention the fact that their story could attract a wider readership if more readers saw themselves represented in it, and I might point out that their work is more likely to stand out if it's not confined to an over-saturated market of white-guy stories.
But the best reason to write greater diversity into a story, in my opinion, is that it reflects the diversity of the real world. Even if your story is set on another planet or in a fantastical universe, your readers are right here and they have to live in the world as it is, with all its diversity. Fiction can let each of them tell their stories.