So far as I'm concerned, Christian will never be a main event champ in the WWE. The reason is that he crossed the line to TNA and did well for them (as opposed to Jeff Hardy who crossed the line and did badly for TNA in the past.) Christian has, in Vince's mind, been branded a traitor. So I'm afraid I'll have to eliminate Christian from this list.
'Taker has already been eliminated for reasons already stated. Besides, it's not as if there's a great deal more he can accomplish as far as accolades are concerned.
John Morrison, Kofi Kingston, and Drew McIntyre are all good solid possibilities, though. If the WWE wanted to give him something else to tie into the "chosen one" gimmick, then I'd say McIntyre, more than likely through some form of corruption/favoritism. In Kofi's case, I'd say he's the least likely on this list of three. He's great in the ring, but the WWE has already shown they're not keen on giving him a long term push. Besides, he's too small for the WWE's liking. If he were to do a heel turn and become a major cheat, that'd be his only chance. For JoMo, I'd think he'd be a solid possibility if the WWE wanted the next triple crown to be a face. He'd also be the best for a good emotionally-charged overcome-all-odds type of story line.
Then there's Santino. I'm going to say that there is more possibility for him than one may think. I'm not saying for the next triple crown, but surely for a triple crown before he retires. It would likely be a very short reign, fallen into "by accident" that sort of thing, but it could be done and would be very entertaining.
As was mentioned there's also Goldust and William Regal. In regard to Goldust, I think he's past his time and any chance to gain a main event championship has passed him by. I like Goldust and think he's a great entertainer...but he's had his time in the sun. The same with Regal, although of all people, I think Regal could make a comeback if the WWE would allow him to. Wait until they actually make a William Regal DVD set. Then we'll see...