I like some of the ideas here, but I think structurally it should mimic The Lord of the Rings in more ways than geography and population. In particular, the story events Tolkien created are a cascade of choices made by the principal characters: because A, then B; because B, then C; because C, then D; because D, then E, and so on. Samwise makes mention of this continuity when he observes that Frodo carries in his glass the light of the star from the tale of Beren and Luthien.
A good specific example is Boromir: because he fell victim to the temptation of the Ring, the Fellowship was sundered; because Merry and Pippin were carried away by Orcs, they were brought to Fangorn Forest; because they were brought to Fangorn, they roused the Ents; because they roused the Ents, Isengard fell; because Isengard fell, Rohan was free to rescue Gondor; because Rohan was free to rescue Gondor… and so forth. (You can follow that chain of choices backward through Boromir, who was sent by Denethor in place of Faramir; and Denethor chose Boromir because Faramir was befriended by Gandalf… and so on.)
Choice is everywhere in the stories. Frodo chooses to bear the Ring. His friends choose to accompany him. Almost nobody in the story is passive, waiting for things to happen.
I think any worthy sequel to The Lord of the Rings would adopt a similar structure, based on a cascade of character choices. For instance, the Dwarves shouldn’t just decide out of the blue to re-take Moria; they should be seeking after Moria’s wealth because, since the destruction of the One Ring, their wealth-generating Seven Rings have failed. This makes their invasion of Moria a direct consequence of the end of the Third Age, rather than an afterthought. Perhaps the Corsairs don’t just ravage the Grey Havens for the hell of it; instead, they’re retaliating for assault on their ships by the Army of the Dead.
The quibbles I have with the plot you have established is that you seem to be pushing characters into position because that’s where you want them, rather than because the characters have a personal reason to be there (Legolas visiting Gimli in the Glittering Caves and they happen to run across a Dwarf camp, for instance). Tolkien didn’t seem to care much for coincidence in his stories. At the Council, Elrond observes that all the Free Peoples of Middle Earth converged on Rivendell seeking advice: the Dwarves because Sauron had sent emissaries to the Lonely Mountain, the Elves because Orcs had rescued Smeagol, the Hobbits fleeing with the ring, and Men because Boromir was asking about the Vision. They all had legitimate reasons to choose to go there; they were not summoned. (Most of them, in fact, were spurred to Rivendell by Sauron’s very own choices, emphasizing that evil contains the seeds of its own destruction.)
Also, you bring in foreign lands little mentioned in the trilogy — this means it’s difficult to build them into that structure of cascading choices I’m talking about, since we don’t have any good idea what choices those people made or why they’re in the story now. About Khand we know little; there seems to be no particular reason for the Mouth of Sauron to be there, at least of which we are aware. It might be more satisfying to say, “The Mouth of Sauron fled to Harad and tried to raise another army; but because of Sauron’s previous lies, the Mouth was rejected and Harad made peace with Gondor; thus, the Mouth went to Khand.” It gives him a reason, a motivation, based on the history of previous choices.
I hope this makes some sense. 
