In the events of the Domination offensive storyline, Lor'themar Theron tells the player that he was considering old alliances, and Varian tells Jaina after the Purge of Dalaran that he was in talks with the blood elves to join the alliance. It is likely that after the events transpired, the conversations have stopped altogether, but maybe it has been merely a setback.
So, If Lor'themar goes through with his plans, it's assumable that not all blood elves would follow with his politics, so there would be a schism within the race. For gameplay purposes, this could mean that the blood elves become a neutral race, such as the pandaren. This would leave the horde with one less unique race, unbalanced.
So, in this scenario, which race should be added to the alliance, and why? My choice would be Ogres: established lore, it wouldn't be too much of a hassle explaining their involvement with the horde. Problem getting through doors? add a crouching animation when colliding with door frames!
Edit: Remember folks, in this speculative scenario, the blood elves go neutral, so they would be available to BOTH factions just like pandaren are (would you expect ALL blood elves to defect to the alliance?) As such, the horde would need another unique race for balance. On another note, this speculation assumes that current blood elf players would not be affected by this, only new ones would. If a horde belf wants to defect, he just would have to race change like anyone would.