It all depends on who the WWE and World Heavyweight champs are. If Orton is still the champ, and Undertaker has it by then, then chances are they'll have a face win to challenge Orton. If a face has beaten Orton, then a heel will win, but lose the match at WM.
Some options, and explanations, in no particular order:
1. MVP - Has a shot, but a very small shot. Instead, I see him winning the Money in the Bank. That's far more likely in my opinion, but he's got a shot.
2. Mr. Kennedy - The only way this will happen is if a face is the champ at the time (no matter who it is: Jericho, HHH, HBK, or Hardy). Kennedy will most likely lose at Wrestlemania, but win afterwards at some point. He's been in line for a push and this would do it, but even if he loses the Rumble, he'll be in the final few.
3. Edge - To set up Undertaker/Edge at Wrestlemania.
4. Jeff Hardy - Definitely has the possibility. They're gearing him up for something, without a doubt. Plus, this fits, as they could have Orton still the champ, but lose to Hardy at Wrestlemania.
5. HBK - The WWE doesn't forget who got what reactions in their Royal Rumbles. Last year, the crowd went APESHIT when it was down to the Undertaker and HBK, so they could possibly have him win the rumble and take the belt from Orton at WM. The biggest knot in this chain, though, is Shawn's work schedule. He's been taking a more lax approach, so I don't know if we'll ever see him as champ again.
6. HHH - Not very likely, unless Orton is still champ and they don't want to push Hardy or Jericho. Still, he has a shot of being the one to take the belt off Orton.
7. Jericho - While I don't think...like most people, it seems...that Jericho will be the main event guy for the next year straight, there's a possibility that he'll win the Rumble. More than likely, though, I could see him taking the title from Orton at the Rumble, Kennedy winning, and having Kennedy/Jericho at Wrestlemania, if Jericho is involved in it at all. Its too early to see where they're going.
People that, in my opinion, have no shot at winning...aside from the most obvious things like Super Crazy or Mike Knox...
1. Matt Hardy - If they're being so hesitant to give him the United States title, they won't push him to the main event status that quickly. By next year's rumble, its a possibility, but 08 isn't going to be his chance. Great talent, a little tweaky on the mic, but he'll find his place after a US title reign.
2. Kane - Alas, the guy's a great performer, but he's gotten stale over the years. He was once the most dominant Royal Rumble competitor pretty much ever, but that is no longer the case. I could DEFINITELY see him being ECW champ by Wrestlemania, but not by winning the Rumble and getting a shot at it that way.
3. Umaga - Heels don't normally win the Royal Rumble, especially if they're someone like Umaga. The only time I can think of this happening is when Yokozuna won back in 93, I think. He's got a spot in the main event, but he isn't the type of guy that wins the rumble.
4. Khali &/or Big Daddy V - No shot whatsoever. I hope nobody even suggests them. They will be the big guys to toss out at best.
5. CM Punk - The only way I can see this happening...literally...the ONLY way...is if Edge is currently the champ on Smackdown, Punk challenges him to help shift him over to the Smackdown roster, but he loses at Wrestlemania. Punk's got a bright future but he's not in the right position to dominate the rumble and win a belt at Wrestlemania.
6. Lashley - His lack of mic skills puts a damper on his push. Lashley should, and hopefully will, be in the Intercontinental scene around this time and not the main event. I could see him competing in the Money in the Bank match, too, but not winning it over MVP.