I can't think of anyone currently in NXT this would really suit, with only the slightest possibilities being Kashius Ohno and Leo Kruger (just based on size for Leo) but I think it's a cool idea. Carlito won the US title in his TV debut, Santino won the IC title in his, Gail Kim won the women's in hers, and there have been several people who have won the WWE or WHC within their first year, so I don't think it's that far fetched an idea.
If you started running vignettes for the guy a month or two out from RR (but didn't say when he was going to debut), had him come in at like number 20-25, and clean house in a convincing way, I think the fans would eat it up. The most important thing would be that unless he's a heel you would want to make sure the last people still in the ring that he eliminates aren't anyone that people are desperate to see win it, like you'd almost want to fill the ring with guys that the fans love to hate such as Cena, Khali, Miz, Big Show, etc.
Whoever they did it with would still have 2 months to build their character up to Mania, and if they challenged for the WHC, it's not "really" the main event anyway between the WWE title and the Undertaker match, so I think it would just be another big element for the Mania, I know I'd be interested to see it.