In some cases this would be easier than in others. Giving draenei a playable broken appearance (seen above) would require a lot of work - as you can see, the model is outdated and is even on the wrong skeleton for a draenei. (It's built on top of the current tauren skeleton.) But allowing a player to choose a Dragonmaw, Blackrock or Mag'har orc is as simple as giving them more options in skin tone.
There would be other possible positives from making these elements possible. For instance, allowing Grimtotem face marks and Wildhammer dwarf face tattoos could later lead to them being available for other races as well - orcs have a history of decorating their faces with skull masks, as an example. Furthermore, while we're redesigning the models, it would generally be useful if more skin tones and hair styles existed for all races, especially humans. I don't expect the fairly limited variation in terms of faces to allow someone playing WoW to truly recreate the variety of people who exist, but some better variation in terms of skin tone and hair style could at least approximate it better.
To some degree, this makes the human redesign the trickiest - we know what humans look like. We see them every day. Most of us reading this are human. A few of us might be sapient computer systems developed in the 1970's by the Soviet Union who used psycho-war implantation techniques to decant their consciousness inside a body grown for the purpose, but that's a minority. In general, we're human, we know what humans look like. Giving players who choose to play as human more tools to look like the human they want to is a good thing. The same goes for pretty much any race in the game, really - more variety for night elves and blood elves and gnomes and trolls (trolls in particular have been shown to be an incredibly varied race, and some of that variety should be incorporated) - races that haven't been shown to have a sub-race as such should still get variance in their skins.
I'm bringing this up because, so far, I'm both impressed and a trifle shocked at how good the new models have looked, and I want to see that extended. Forsaken players have only recently seen their new female model, for example. I really like what they're doing with them, although in some cases I feel like bones are sticking out where bones wouldn't go (I'm not an anatomist, so I may be wrong) - but what I'd like to see is forsaken get more variety in head and face models, extended from what we had in vanilla. More stuff fallen off or out. More stuff decayed or withered. And maybe even a few weird oddities, like giant high elf ears that suggest this forsaken was one of the ones created by Arthas' attack on Quel'thalas (or worse, got parts from one of those victims) to play up the horror of the forsaken.
Now is the time to take the extra step and make players gasp. Give us those things we never expected we'd get - real options to help us set ourselves apart from other players.