Sid vs. Taker, I believe, was always planned, just not for the belt. The ending of the Rumble was planned to set up the #1 Contender's match at Final Four, and was believable since Austin and Bret had been feuding for months prior.
I've always been curious as to what the original plans were for Austin if they'd gone ahead with Bret vs. Shawn at WMXIII.