Here's an idea I had: Rare race/class combinations would basically allow players who really want to play as a certain race and class to put forth the effort to be allowed to do so without completely opening the floodgates to swarms of Undead paladins, Draenei rogues, Pandaren death knights, etc. Through in-game accomplishments, you could earn a token which allows you to roll a character of one of the race/class combinations below.

List (Very general suggestion):
Human: Druid, shaman
Night Elf: Paladin
Gnome: Druid, shaman
Draenei: Rogue
Worgen: Paladin, monk, demon hunter
Pandaren: Druid, warlock, death knight
Orc: Priest, paladin
Undead: Druid, shaman, paladin
Tauren: Mage, warlock
Troll: Demon hunter
Blood Elf: Druid
Goblin: Monk

Unlocking: Players would need to acquire a special token to create a character of a rare combination. Once you acquire the token, it will enable you to create one rare race/class combination character, or race change an existing character to a rare combination, and doing so will consume the token. Sources of tokens could include (Again, these are just ideas):
• Completing “Ahead of the Curve” in any raid.
• Completing a PvP season in the top n’th percentile of arena/RBG players.
• Rare drop from miscellaneous sources, such as world quest emissary boxes.
• Tokens would not be bind-on-pickup, and can be bought and sold on the auction house. This would allow players who don’t raid or PvP at a high level, but have plenty of gold (Or some money for WoW tokens), to acquire the tokens. To prevent speculation shenanigans, tokens will become soulbound once purchased through the auction house.