I know anecdotally that editors can be quite thankful when they do not have to remind an author that people have different ethnic backgrounds. Characters should also. Recently, as the research phase for A Song Heard in the Future came to a close, I found that is was not going to be difficult to include a range of heritage.
Given that almost any (every?) story set within the world of Greek mythology and worship will be a Mediterranean tale, the likelihood of an all-majority cast seems very, very low. That said, several of the major characters are not actually Greek.
The first king of Thebes was initially from Phoenicia and one of the kings of Thrace was a Libyan immigrant. Pygmalion (or Pumayyaton), the famous sculptor, was a Cypriot. The more-traveled characters may even have heard of a Titan by the name of Gadeiros from what would become Spain and Norax, a hero on the island we call Sardinia and the Greeks knew as Ichnūsa.
The biggest revelation to me regards the ancestry or a demigod famous for Twelve Labours. His great great grandparents were Ethiopian royalty. Is the world ready for a Hercules who is at least one-eighth black?
There is another story about publishing I’ve heard. I won’t repeat it here because it’s absurd and offensive but it regards the readership of science fiction and fantasy. We’ve moved beyond that, right? In this case, I’m hopeful that any future editor of Song will be intrigued by the prospect.