Thinking about this quite often and polishing my idea about over the past year or so and I find hardly any arguments against it.
I brought this up on the wow forums 3-4 years or so and it ended badly so this time I want to try here first.
So thea suggestion is as follows:
This could happen as an expansion feature at max level or at level 10 or somewhere along your leveling journey.
Lets say you play a Tauren warrior. After a while you do a storyline specific to your class and racial combination.
This storyline would be something about the legscy of warriors among taurens. After the storyline is over you become what the taurens warriors actually are called within the lore. That is a "Brave".
This could reward you with a suitable cosmetic armor. And most important an iconic ability specific to Braves. In order to not mess up balance it would for example replace Mortal Strike with Pulverize which has the exact same numeric value as Mortal Steike, but just with a very taurenlike ability and icon. It could be like slamming down a totem on your enemy.
Braves and other names like these is something we see in the game all the time.
Other types of combos would be:
Tauren Shaman = Spiritwalkers
Troll Shaman = Hexxer
Nightelf Rogue = Sentinel
Nightelf Hunter = Archer
Dwarf Hunter = Rifleman
Orc Warrior = Blademaster
Troll Priest = Voodoo Master
Troll Warrior = Berserker
Troll Hunter = Shadow hunter.
Human Warlock = Necromancer
Undead Warlock = Lich
Etc etc etc..
Many of these might not be the right names. I am just throwing out things here so you get my point. Blizzard I would assume would have no problem coming up with cool names for all combinations.
It would give people motivation to level more alts to try out the different types of combinations.
Thanks.