Theoretically, if Hou Yifan wins World Cup 2015...

Hey it could happen, unlikely but could.
How unlikely, you ask? With current FIDE ratings (Carlsen: 2853, Hou: 2671), there is a difference of 182 rating points between the two players. This means that Hou's odds for beating a game against Carlsen (not adjusting for white advantage) is about one in four. For a championship match of 12 games, this would mean that Hou would need to get more than six points; if this were translated to six victories, the odds of achieving this would be about one in four thousand.
So, if Hou Yifan ever were to not only reach but to win Candidates from her current 72nd best player position, she would have about the same odds to beat Carlsen as to roll five sixes in Yahtzee with one throw. I'm not holding my breath for this one...

She'll only have to beat Leitao, Mamedyarov, Harikrishna, Caruana, Kramnik & Nakamura (or not since both finalists advance to candidates tournament).
And then some of these guys + Topalov and Aronian again. And then Magnus. At least 4 times. Fingers crossed. I bet 1€ on her. May become a billionaire next year, guys.

At the moment is Muzychuk. There's a match with her coming next, I think.
The match was originally supposed to be next month but is now scheduled for next March in Lviv, Ukraine
No one would become the greatest ever by doing less than players like Kasparov and Lasker did, i.e. being the clearly best player in the world for decades. Hou isn't even top 50 yet so she has a long way to go before being best in the world for years.

Wait a moment - 200 rating points difference doesn't mean that her chances of beating him in a game is one in four. It means that she has 25% success expectancy overall.
This may be, a couple of draws and a win out of eight games, or three wins and three draws in 18 games, or one win and five draws in 14 games.
If she really had one chance out of four to beat Carlsen, maybe she was actually the better player (in case the other three games were draws for example).
...qualifies for the Candidates, wins Candidates, challenges Magnus and win the match against him (and meanwhile wins in a match against M. Muzychuk) - can she be declared the greatest chess player of all time?