Lets look at what we know so far......
There are different levels of allies, and without knowing what those are, and its effects, I am going to just pass by this for now.
It is also through rumored consensus that you will have to take your normal foc required slots first, and then you would be allowed to take 1 HQ and 1 Troop for your allied detachment. After the 1 and 1 requirement is filled, it allows you to take 1 Fast Attack, 1 Elite, and or 1 Heavy Support.
I have heard many people already complaining that 1 detachment of Grey Knights will be in every force. Grey Knight Terminators are pretty tough added to some armies, however I think looking at Grey Knights being every ones allied detachment is a little bit overblown and exaggerated. I don't think this will be the case.
What I think people will start looking for the biggest bang for your buck with allies, and its not going to be Grey Knights. Its going to be the Imperial Guard. Why?
I think the Imperial Guard will be easy for almost anyone to ally with. Next the Imperial Guard offers things that most allied forces will not be able to offer... to really make a huge difference and get the most for a single foc choice. 1 HQ is alright, but what you can get out of a single foc choice is phenomenal. The infantry platoon and vehicle squadrons.
An infantry platoon is a single choice, but can really bring along 6 tanks, a mass number of infantry, and special or heavy weapons. Its very flexible, and will mesh with most army style lists. Flamers (great for defensive lines), heavy weapons, combined or separate infantry squads, etc are just some of the many bonus's other armies will be able to take advantage of. Best of all, you get all these options for a single foc choice.
Want some flyers? How about a squadron of 3 Vendettas. I know we do not know how the rules will go down with vehicle squadrons yet, but if you really want some bang for your buck, 3 vendettas, or 3 leman russ's can really add to your firepower. You can't overlook the Hydra Flak Tank either... with flyers coming, flak tank batteries could easily become a staple.
A single Infantry Platoon consists of
1 Platoon Command Squad
2-5 Infantry Squads (which can be combined)
0-5 Heavy Weapon Squads
0-2 Special Weapons Squads
0-1 Conscripts Squad
Fast Attack Vehicle Squadron Units
Valkryie Assault Carrier Squadron 1-3
Vendetta Gunship Squadron 1-3
Armoured Sentinel Squadron 1-3
Scout Sentinel Squadron 1-3
Hellhound Squadron 1-3 (Banewolf and Devil Dog variants)
Heavy Support Squadron Units
Leman Russ Squadron 1-3 (many different varieties)
Ordinance Battery 1-3 (basilisks, and more)
Hydra Flak Tank Battery 1-3
Getting so much out of so little foc slots is really an advantage that many people will start taking advantage of. Right off the get go, can you see Grey Knights who have a serious issue with range, get all they need out of a single foc slot with allied forces? There are many combinations we will start seeing, and I believe that the Imperial Guard will be at the heart of it.