First, note that this discussion is aimed at those who have participated in our campaigns before -- some of the topics are a little confusing if you haven't. Don't worry about it if that's the case.
So at the start of many of our campaigns, some of the armies start out allied. That limits who you can fight on the first round, and sometimes onward to later rounds. For example (in this campaign) Gondor and Rohan wouldn't attack each other on the first round. And certainly main Gondor (Minas Tirith and Ithilien) wouldn't attack South Gondor (Pelargir, Anfalas, Dol Amroth) and vice versa. Not even with the deception and stuff that the Eye had in place (Denethor's madness; Wormtongue's work on Theoden).
Similarly, in spite of a rocky start at the Battle of the Five Armies 75 years or so earlier, the Dwarves, Men, and Elves around Erebor are friendly and allied. But Rohan isn't allied with them -- in fact, they know little of Dwarves, and are very suspicious of Elves (cf. Eomer's suspicions of Galadriel and Lothlorien). And Gondor might be more concerned with nearer foes, but the madness of Denethor might easily be turned to considering that uniting the men of Rhovanion under the banner of Gondor would be a good thing to hold back the Lidless Eye.
On the flip side, it's widely established that the forces of Sauron fight each other all the time, even within the borders of Mordor (cf. Cirith Ungol and Shagrat and Gorbag). It takes his direct intervention (in the person of his Nazgul) sometimes to prevent that.
So what I'm thinking right now (and I'll let you know if and as that changes), is the following.
- any Gondor armies and any Rohan armies start out allied.
- Dale/Rhovanion, Mirkwood Elves, and Erebor/Iron Mountain Dwarves start out allied.
- any Evil army that has at least one Nazgul starts out allied with any other Evil army that has at least one Nazgul. If you have no Nazgul you are independent -- so the Haradrim (if they have no Nazgul) can attack Khand, or the Easterlings, or other Southron forces, as well as Mordor or the usual suspects (Gondor, Rohan, or the West).
As an aside, I've been thinking about the thought that more than one player may want to purchase the Witch King for their army. But there is only one Witch King. The solution is easy -- if more than one Mordor army wants the Witch King, the one who pays the most points for it gets to claim his is the real McCoy. Anyone else has one of the other Nazgul. Mighty, terrible, etc. etc., but not Mr. W. King. This won't change anything in what you are allowed to take -- it's just about pride and boasting.