Disclaimer: This theory requires that another race also receives a sub-race, so that the Horde could get a new race as well and not just the Alliance. I do not see that as an impossibility at all. Both races would in that case require very little work compared to entirely new races, as they already have models, settlements, lore, and so on.
Expansion V is probably themed around the return of The Burning Legion, and Alleria Windrunner and Turalyon are 'needed for what's coming next', according to Metzen. The Army of Light will possibly be formed to fight the Burning Legion. Turalyon might become the military commander of such an army, considering his experience in dealing with the legion, and his hatred of all things evil.
Alleria Windrunner, however, is famous for her ferocious loyalty to the Alliance and hatred of the Horde - while it is likely Turalyon might answer a greater calling than the faction conflict, Alleria probably won't. Her calling is to reunite her people, and restore it to its former glory. This opens up an endless array of possible story lines involving the elves of the Eastern Kingdom and the Windrunner sisters, on the side of the main story that deals with the legion.
New neutral player race: High Elves.
Horde: Sin'Dorei (Blood Elf).
Alliance: Quel'Dorei (High Elf).
- Alleria Windrunner returns and forms the Quel'Dorei faction - its primary purpose being to restore the full might of the High Elves and 'cleanse' their brethren, the Blood Elves. Extremely loyal to the Alliance.
- Alleria sets up bases along the coastal stretch of land bordering Quel'Thalas and Lordaeron, sometimes referred to as 'Northeron'. From there, she watches over Quel'Thalas and welcomes any blood elf who wishes to repent and join her. (The name Northeron is said to have referred to another region, but let's stick with it for now.)
- Level 1: Player High Elf meets Alleria and Vereesa in Northeron. Alleria is infuriated as Vereesa refuses to leave the Silver Covenant for the Quel'Dorei.
- Level 10: Player meets Alleria and Sylvanas in a quest, and is faced with an ultimatum. Joins Horde or Alliance (Blood Elf or High Elf).
- Level 10-20: Player quests through the Ghostlands or Northeron.