I think it would be cool if Kennedy win the match and then cashed it in later that night immediately after either Triple H or Cena won the WWE title match. This way, he would make true his promise that he would headline WM24. He lost the briefcase to Edge due to injury but they could let him win and finally give him his main event push. I don't think this will happen, as Hardy will probably take it, but it would be a little unpredictable at least.
In this case, some might recall Hogan coming down and taking the title from Yoko after Bret lost it at WM 9. However, this makes great sense from the standpoint that the winner of MITB can challenge at any time, and what better way to make a lasting Wrestlemania moment, than by adding another match to the card as the show is literally about to go off the air. It would be a great addition to a card that is shaping up to be not so memorable, and it would work for either a heel or a face depending on who ends up champion at the end of the night.
If Orton or HHH win(and I honestly hope Orton retains just so he doesn't come off looking like a stooge and less important than Cena or HHH), Jeff Hardy is the obvious choice to win MITB. Hunter or Orton go over, the 2 losers go the back, and mid celebration just as we are about to go off the air, out comes Hardy with briefcase in tow. In this case, it has to be a real match and last for a little while, and cannot be a quick squash by Hardy capitalizing on the Champ being tired and worn down, or you risk heeling him. However, when Hardy wins neither Orton or Hunter really lose face, b/c they play it off as a fluke the next night on Raw. Both have history with Hardy, so logically the feuds are already built in. In HHH's case though, he doesn't have to be full heel when he loses to Hardy at Mania, as the feud with Hardy will itself be the vehicle for the full turn.
If Cena wins, Kennedy is the logical choice to win MITB and come out mid celebration and beat Cena. However, he can heel on him and beat him cheaply the way Edge did. I'd still prefer a full match, but Cena doesn't have to lose face either. In the 3-way dance with HHH and Orton, they can work a reinjury of his pec, and then have him come back and still win the title. He then his loses to Kennedy and that can be written off due the next night on Raw due to his injury, and also lead into a nice long running feud between the 2. I'd really like to see this feud anyway as both are good on the mic and I think they could run with a war of words for a while before there actually had another match. The only problem with Kennedy going over Cena, is that WWE wants Cena as a babyface, and I see Kennedy getting huge pops from the crowd in a feud with Cena and beating him at WM. This is why if they are going to pull this off, I think it will be with Hardy.
Either way though, if the winner of MITB is going to do a surprise challenge after the Main Event, whoever it is has to win the match. Otherwise, the whole challenge is pointless, therefore killing off the specialness of the MITB(which I am not a fan of to begin with)and does nothing to further any possible feuds with the MITB challenger or the champion.