Had Smaug Lived to See LOTR...

[Legate of Amon Lanc]

There is a rather good outline by Gandalf himself, mentioned in the Unfinished Tales, in a part which, actually, was originally intended to be a part of the Return of the King (Gandalf telling that to the hobbits during the "peaceful rest-time" after the coronation of Aragorn):

Originally Posted by Unfinished Tales, The Quest for Erebor

"It might all have gone very differently indeed. The main attack was diverted southwards, it is true; and yet even so with his farstretched right hand Sauron could have done terrible harm in the North, while he defended Gondor, if King Brand and King Dáin had not stood in his path. When you think of the great Battle of Pelennor, do not forget the Battle of Dale. Think of what might have been. Dragon-fire and savage swords in Eriador! There might be no Queen in Gondor. We might now only hope to return from the victory here to ruin and ash. But that has been averted – because I met Thorin Oakenshield one evening on the edge of spring not far from Bree. A chance-meeting, as we say in Middle-earth."

I think the idea was that Sauron would eventually recruit Smaug to his cause and use him as a vanguard of his army which would allow the Easterlings (not hindered by the Men of Dale and the Dwarves of Erebor) to pass through the Grey Mountain narrows, cross the Anduin close to its beginning (where it was still possible) and pass the mountains to Eriador - resp. retake Angmar and basically re-launch the Angmarian campaign all over again. All the other things mentioning that subject seem to point towards this direction (and I have been looking into this topic rather deeply, actually, as I used to be interested in it a lot).

EDIT: crossposted with the two above