Actually, I think diversity is pretty good to focus on. If the bulk of your players (I suspect they are mostly male and mostly straight/lesbian) are not comfortable with their avatar character, or the NPCs they are given the option of interacting with, then your audience for the novel immediately drops and, if you didn't warn them, you may attract negative reviews and ratings. So an important question to research is just how open minded your audience is.
There are also technical reasons why racial diversity is important. Sometimes skin tone is the only way you can work out which bits belong to which character... ;-)