Hmm, since I write historical fiction making a racially diverse cast is difficult of course. But we shouldn't forget that a 'different race' has always been subject to fluctuation. For example, in the Victorian era you were already a different race in Britain if you belonged to the lower class. Not to even begin about Irish or Scottish or European immigrants. Who were all decisively 'white' at one point in time.
Perhaps another example: hispanic/latino people in America. It was not that long ago that being Spanish meant you were properly European 'white', and I still don't consider to be of Hispanic descent to be that different other than perceived through a separate culture (or in some cases things like distinct African heritage and Mayan heritage).
So when you call for a cast of 'Diverse characters' you have to be careful not to label too many groups in that different class of 'diverse worthy'. In American series, I don't think that including proper black characters with good backstory and character development should be labelled diverse. Since I think it's pretty much the norm for America to have a share of African, African-American, Hispanic, Asian, and European characters.
In the same vein, a historically accurate story may be very diverse just by pointing out that in that time, for example, Irish, Scottish, Women, Workers and white immigrants were different groups, without ever having to introduce those whom nowadays considered diverse in American pop culture. (Still I agree you can manage to do people of colour as well, if you understand the circumstances how and why they would be in that time setting and place: freefought slaves, service personnel with more backstory than they let on, or someone on the run for example.)
So I think my argument is less diversity for difference's sake, and more to show how the world functions in terms of classes, roles and social structure.
In that same vein, you can put aliens or orcs in the place of such a group, and make it so that all human races are the 'priviliged'/'standard' class. Then that would make it in my eyes more diverse than an all-human cast of heroes, since in your world aliens or 'monstrous' races have taken up the banner that African-American, LGBT, or Hispanic have to now - and women, Irish, Scottish, Italian and Slavic people have done so before.