[The Squatter of Amon Rûdh]

I was thinking of the fact that Saruman was utterly outnumbered by the hosts of Mordor, with no buffer zone between himself and his rival (if Rohan is removed, which seems likely).
I'm sure that Sauron wouldn't have given so much of himself to the Ring's making if he didn't gain some sort of advantage over other users and there are a number of references to the Ring's having but one master. Just look at the arrogant confidence with which he announces through the Palantir that he'll send for the Ring; it must have occurred to him that Saruman might try to challenge him, yet he can't even be bothered to deal with the matter personally.
One could also see Galadriel's queen speech as a fantasy created by the Ring, akin to Sam's of making Mordor bloom, combined with a desire to frighten Frodo out of trusting the Wise unquestioningly (I'm sure that if the Ring was trying to turn her she was aware of it).

It would be one hell of a battle, though. If he hadn't played his hand too soon, Saruman would have been in a position to destroy Rohan unimpeded, which in turn helps Sauron to reduce Gondor and leaves a straight contest between Mordor and Isengard. Not a pleasant thought, but I'm still fairly sure of the outcome.