It would be really nice if Death Knights got the same number of options as Demon Hunters and Evokers, since these are the only three classes that have special selections for their chosen race model. The easiest start could be to add more eye colors, like red and green, to add to the spec fantasy.
The old zombie skin colors as well could be reworked and expanded upon too with the addition of the ‘dark ranger’ skin & eye color that the elf races got.
The class / race of Death Knight seems a little outdated with their customizations compared to the new two hero classes. It would be a really nice refresh to modernize the effects and models of the game’s first hero class.