From the start of the game its either been the factions going after the same goal (enemy of my enemy) or at war. They don't live together, the game isn't based around mercs teaming up to defeat super bads, its this way since the get go. It might not be what you want to hear but if you want to team up with your buddy whos in a different faction so bad create a character in that faction and have at it. This isn't needed. It is a matter of convenience for folks that want it "their way, and here's why" and there are a lot of people that don't want it that way. Having the populations go up and down have always been a thing. Look for server merges.
Edited to add: I guess the alternative is to have everyone go horde so there is a huge blinking neon sign for blizz devs to look at and help find solutions. Homogenizing server groups and PVP servers was the start of issues. There use to be a since of pride in large pop Alliance servers and Horde servers that kept them full and kept faction pride alive. They have watered down all of that to a point it doesn't matter.
- - - Updated - - -
At that point I still don't think allowing factions to merge in content is the answer. It's not something I will be on board with. I play and have played both factions, I am not having these issues on either.
- - - Updated - - -
In all honesty I'm not sure there are that many people who actually want this though. I'm not seeing the normal blasts of posts.