So with the cross faction grouping and cool things to come what is the Alliance and Horde called now?
Its obvious the Alliance is the greater force compared to the small rag tag beast monsters of the Horde. So I’m guessing it will still be “FOR THE ALLIANCE,” instead of the barbaric Horde.
I personally think the Horde will benefit from this and learn some civility and manners but what do the Alliance get out of this on a lore level?
FOR THE ALLIANCE!!!
… The Fabulous Furry Force!!!
With new and improved Gnome guest stars.
For Azeroth?
I dunno.
Something’s not quite right.
Are you SURE you aren’t a paladin?
For the Legion!
Err uh… I mean the Alliance.
Yep.
FOR AIUR!!!
(yes this is a complete sentence)
For the players! That’s who!
I am not that quite divine and I am more of a woodsman warrior and inservice to his majesty, King Wyrnn the great. A proud loyal soldier and protector of the great city of Stormwind. 
For the MOOOOOOOO!
/moo 
I don’t care what side you’re on. I’m going to equally hand out butt whoopings in PvP regardless. Equal opportunity demoralizer.
Probably just a bland: for Azeroth¡
We’re friends now. The friendliest friends who ever friended all the way to Friendsville.
That’s not very friendly of you to say friend.
You get access to everything the Horde has that might benefit you.
We are all Alliance now.
For the pizza!!
But is it the one pizza. To rule them all!
As long as you recognize who your Superior’s are, its all good friend. 
Let’s get our hair done!
I always yell “For black lace panties!!” When I charge in.
It will be called The Horde, as the Alliance has become too small to sustain itself as a faction. Your kind will be assimiliated into the Horde War Machine and your former territories will benefit from the protection offered by the Horde’s collective might.
LOK’TAR OGAR!
It’s still “FOR THE HORDE” I’m pretty sure the Alliance was a ded faction, with cross-faction it is being raised from the dead, thus they are now forsaken. Still just Horde.
Spoon. For the spoon.