Well where many people have mentioned Kofi, his breakthrough came at Survivor Series, but we all know what came of that since then. Problem is, not everyone breaks through at Wrestlemania, but it is the key PPV where a breakthrough is expected, as Lariat said the likes of Hogan, Michaels, Austin all got their key breakthrough at Wrestlemania whilst others like Undertaker, Mankind, The Rock, Triple H brokethrough at PPVs like Survivor Series or Summerslam.
That being said, I do agree that Sheamus will the key guy to breakthrough at this year's Wrestlemania, he has been primed and set up to do so. A lot of people were shocked in disbelief at him getting the title but he grew on us and we were getting use to his title run but this match with Triple H is the essential one to make or break him as his matches with Cena and Orton didn't prove much except that he was "lucky" and we only got to see a stint of him in the Chamber which started to get his ball rolling but unfortunately he was forgotten due to his sudden elimination with three other guys. So this match with Triple H is a big moment for him and for me, he has to win because currently on PPV, TLC = Table Win, nothing big, Rumble = DQ Win, more about Orton's story, Elimination Chamber = Eliminated and didn't take out anyone significant, so his track record isn't one for us to back so much like everyone has said Sheamus' time is Wrestlemania and hopefully his place will be solidified as a Main Event Player.
As for others named, Christian, he kind of didn't really break through but snuck in through the open window in the glass ceiling, he just hasn't had a big win outside of E&C days, before he left in 2005 he was facing Cena and much like Jericho, because he never got the big victory off of him, momentum was lost. If he wins MITB, I don't see it as a breakthrough because he needs a significant feud on Raw to show he is able to hang with the big boys. Carrying ECW for a whole year is one thing, but it's a different game for being a Main Event guy on Raw, give him a big feud with someone like Orton and we'll see his position.
Kofi, as said his breakthrough came at Survivor Series but has since died down.
Legacy, the problem here is that neither guy has really gone far in the past year, they were ousted out of the tag team titles at NOC, their feud with DX led to them going nowhere, they got a win which got cancelled out at HIAC and then they just disappeared off the card until the Rumble. Dibiase got the pin on Orton in the Chamber but he got eliminated thirty seconds after. You see the problem here? They just don't know how to pull the trigger on Dibiase while Rhodes has simply had nothing going for him. While he's been the guy screwing Orton, he's not got any momentum going behind him and I just feel I can't personally back them, even if they do a great performance at Mania. In short, Legacy has been booked poorly and I can't see them doing anything big to break them through at Mania. If their match is a Triple Threat (only rumoured) the only benefit I can see is Orton winning resulting in Ted and Cody to feud against each other. If the past twelve months has seen a big mess up of potential Main Event players, it's Legacy.