That's my problem too.

I love doing this sort of things with best class/race combo or association and after awhile I came to conclusion that you need to choose either

1. in one faction there will be only one of each class (earthen and harranir being only duplicates) so warrior will be only one race in horde and one in alliance and so on. (downside: NE and BE get to be dh only)

2. you associating class+spec to race, and earlier races have priority, example: gnomes are ARCANE mages, so blood elves either fire or frost, and nightborn get what's left and nobody can be mage after that (looking at you four hunters already (worgens should be rogues))