Well, as an African American myself, when I was younger and beginning to write stories, I often had African American characters as my main characters. Because, as a child, these characters were based on people I knew and loved. However, as I grew older, and began to see more of the world and learn more about race and culture and diversity, I expanded my characters to different backgrounds (including, but not limited to race).
I do have different races in my stories, yes, but that comes with doing research and personal experiences. Can't have diversity for diversity sake, after all, especially if the time period and era is generally unable to show it. Of course, that's not to say you can't do diversity in certain time periods. But, again, gotta do some good research.
And I usually base my diverse cast on two types of diversities:
Surface Diversity: This is where you have people from different social groups within one group. A lot of times, people sorta mess up with this because, again, they do it for diversity's sake. That's where we get "token minority" characters, littered with stereotypes and weak character development. When I build my cast with this, I also flesh out similarities between my characters. Even if they come from different backgrounds, what do they have in common? Is it a dream? An Idea? A hobby? Even if they come from different backgrounds, do they have a mutual understanding and respect for one another?
In-Depth Diversity: This is where you have people from the same social group but with different personalities and circumstances . It's easy to think diversity means including different races or sexuality or genders or etc. But it's also diverse to include the same race but with different personalities and different views on said identity. Even if two characters are the same background, do they embrace that background? Did they face the same adversities or was one more fortunate than the other? Do they have different opinions on the same things?
I know I have one comic that includes six African American siblings. That's the entire main cast with a few diverse side characters thrown in there. However, they all have different personalities -- they're all they're own person. And even better, their race isn't immediately tied to who they are as a person. It plays a part, but it's not the main focus.