I also had 6 horde characters with different classes and 6 alliance characters with the other remaining classes, until Allied races was released.
Allied races made me double my characters, making 12 on each side, because some are really cool, and i want to have some classic race - class combos, and some allied race - class combos.
Besides DKs and DHs who only allow classic races , i race changed some of my old characters to allied races, and am now leveling or waiting for release of new allied races.
Im planning to level them all to 120 eventually! (probably in 2 years time). Having more than one active account also helps
Here is the list i plan to have in the future:
Warrior: Kul Tiran Human (new) - Highmountain Tauren (race change from Tauren)
Paladin: Human - Zandalari Troll (new)
Hunter: Void Elf (new) - Vulpera (race change from Blood Elf)
Shaman: Dark Iron Dwarf (new) - Tauren
Rogue: Worgen - Nightborne
Druid: Night Elf - Zandalari Troll (new)
Monk: Pandaren - Mag'har Orc (new)
Priest: Lightforged Draenei (new) - Blood Elf
Mage: Draenei - Nightborne (new)
Warlock: Void Elf - Goblin
Death Knights: Human - Undead (new)
Demon Hunters: Night Elf (new) - Blood Elf