Some other possible opponents for Taker:
Brock
Batista
Punk
Cena
Is Sting, at this stage, a better draw than any of those?
IMO, the longer Sting waits
(provided he wants to do it), the less leverage he will have. As mentioned above, his drawing power isn't getting any stronger. Personally, if they were to go in this direction, it is almost necessary to bring him into the Fed and build some groundwork story around him to get fans interested.
I would love to see them return Borden to the "Crow" gimmick he did around '98. No talking. Put him up in the rafters while making the occasional run-in to the ring to beat the shit out of some guys. Utilize the "smoke & mirrors" special effects. Meantime, Taker could be doing similar appearances. I think the competing styles would compliment each other nicely and make an interesting angle.
In the final weeks before Mania, have Taker in the ring one show delivering a promo with Sting attacking him after making some mystical appearance. The next show, have Taker return the attack on Sting. They could get very creative with the details of this, and the buildup and anticipation could really help carry the match for two workers past their prime - much as it did for the Hogan/Andre showdown at WMIII.