As a mark-out, albeit unrealistic and impossible scenario, imagine if it were none other than Sting. I don't know what Sting's contract situation is in TNA and I don't believe for one second it actually is him, but if the attacker were to be revealed to be Sting: lights go out and he descends from the rafters, that would be incredible. Impossible in all likelihood, but still incredible.
I don't think it will be Kane, he doesn't need a push of this magnitude at this stage of his career. It won't be Cena or HBK, that's even more unrealistic than my Sting scenario. It wouldn't be logical for it to be the SES/CM Punk, they are already established. And it won't be some random young guy like Ziggler, Shad, or Rhodes, they're not significant enough for something like this.
My guess is Drew MacIntyre. Talk of the WWE brass souring on him and his push could be creative leaking of info to the ever vulnerable IWC. I still think he's very much a part of the future in the WWE. What a great heel push this will give him, if he is able to follow through on it. The only other person currently in the WWE I could see it being would be Jack Swagger, but again, he's already established so it would be unnecessary.
It could be someone not currently in the WWE, getting ready to enter and make a big first impression. How's ths for a thought? How about the Briscoe Brothers are coming to the WWE, making a name for themselves by assaulting the Undertaker. Setting up a Briscoe Brothers versus Undertaker/Kane debut scenario?
Unfortunately, it's likely that this is simply a storyline to get Taker off TV for a while to recuperate from his injuries, and that in all likelihood, WWE has not even thought it through and it could be just about anyone.
Maybe they'll just create a big storyline, create a whole bunch of interest and speculation about it, and just never reveal it at all and let the storyline just fade away.
Oh sorry, wrong organization.