Except WoW isn't a World War 2 game. It's faction based fantasy MMO where the factions comprise unique races not available to the other side of the divide. Duplicating a part of one faction to the other undermines the distinctiveness of those factions and degrades the faction barrier.
As for the 'themes' of Blood Elves and the exiles diverging wildly, they have not. Blood Elves and the exiles represent the same theme, generic fantasy elf. That theme is a part of the Horde experience now, just as the Orc theme or the Troll theme is. If you want to play a generic fantasy elf, the Horde is waiting for you.
If you cannot accept the Horde, the 95% Void Elf match is there for you.
The perfect is the enemy of the good. You cannot have 100% of what you want. You should settle for close.