The answer that typically comes to that question is "somebody worse." The most often mentioned by name is Alberto Gonzales.
One reason given for it being "somebody worse" is that the President will be angry with the opposition to Ms. Mires nomination, and will strike out to get even. I.e., spite.
Who knows what he'll do.
If he picked Gonzalez, he would go down in history as a small minded who squandered opportunity after opportunity to make any real difference on the court.
Roberts for Renhquist - Status Quo
Gonzales for O'Connor - Status Quo.
He made a specific promise and must be held to account.
And some of the folks who so characterize the situation say that therefore we should "trust" the President. It's like sombody's handing out crazy pills or something!