Racial customization

The options the existing system offers for visual customization are overall rather limited. However, there are several ways to expand it.

• Body shape scaling: Different body shapes would add another element of uniqueness to the game. Well, considering that WoW already has 13 distinct races (on top of other humanoids like ogres etc.) might make this seem too much, but it's not as if different body shapes necessarily have to be total departures from the baseline physique of a race. Adding scales that enables player to make their models slightly leaner, chubbier or more broad-shouldered etc. should be possible even without breaking the animations or the armor scaling.
• Size scaling:  this would be an extremely simple solution to allow for more differentiation between individual characters. Even a mere difference of 10% between the tallest and the shortest version of a race/gender-combo would already make a notable difference when it comes to appearance
• Unlockable skins and physical features: a very particular modification, but one that might prove overly popular (though tying this to achievements or similar mechanics would limit its prevalence) - the option of picking additional skins or builds that have to be unlocked first. Some examples:

• Fel-tainted for Orcs (Fel Orc look from the Hellfire Peninsula), Blood Elves (Demon Elves from the Quel'Danas), Draenei (Eredar look);
• Titanforged for Humans (like Forged Vrykul), Dwarves (Forged Dwarves), Gnomes (Mecha Gnomes)
• Seaborn for Night Elves (Naga-like skin and features);
• Demonic for Night Elves (Satyr-like skin and features); 
• Undead for all standard races (this of course would conflict with the Forsaken Human and Forsaken Elf allied races).
•  Skeletal for Undead.
• Additional options would be more insulated features like a pair of horns, glowing eyes (for races that don't already have them), a scaled arm and similar details.

A fel-tainted Draenei.