It's a red herring and nothing will ever change my mind about it.
There are many ways to interpret it, but let's be clear that the only person who really knows the truth is Kishi. At beast we can only speculate. It could be anything.
Some people believe that it shows Sakura is still in love with Sasuke and such and blah blah blah. Honestly, that is one reason why it's a red herring to me. It is such an obvious answer to say "Well it means Sakura still loves Sasuke,"but Kishi is not known for obvious answers especially when it comes to the pairings of his manga. I am not sure why to this very day some even here believe that all NS scenes no matter what are always "up for interpretation," but feel that all SS and NH scenes have to be played straight forward. Even SK gets this too.
Some believe, like me, it is a distraction. A stalling agent to keep the reader guessing till the end. It is evident because Sakura is not thinking of Sasuke in a positive light on this one. Looking at Karin for example, she is an extreme case of fangirling Sasuke. Similar to how Sakura was in part 1. Part 2, however, Sakura has shown nothing of the sort. Not even a little bit similar to Karin's behavior. I get people trying to tell me that "Karin and Sakura are not the same" argument which is true, but Karin would be the extreme sense of the fangirling. If Sakura showed even a fraction of that same ideology, then it would be clear to me. Also, because Sakura is different from Karin can also be the same reason why she is NOT fangirling Sasuke still and is just sick of his crap by now.
As well, as Nostradamus brilliantly said, if Sakura thought of Naruto at that very point in time it would be game over for the pairing wars indefinitely.
Some people believe this is the process of letting go of Sasuke and her finally coming to terms with the truth that Sasuke is a bad person or a person that is not for her. Which is just as possible as anything else in my opinion. Of course, there is going to be drama over all of this because it's not going to be that easy for Sakura to just get over it. Otherwise, people won't accept when she does choose Naruto in the end. You have to show how she moves from one to the other and how she realizes that maybe what she once thought was love was just really a shallow crush.
As for the "someone else...," well we don't know exactly. That's the point. She was so ambiguous with it that it could be any of the two boys. I get told it is bad to assume she meant Naruto, but on the flip side it is just as bad if you assume she meant Sasuke. It could be any interpretation until we figure out the end or until Kishi tells us the truth of it.
I just don't get why 540 is the end all and be all of pairings wars development as some are trying to push. People saying it is an obvious answer tells me even more that it isn't that simple. Kishimoto is never that obvious about the pairing development and to claim as such obviously hasn't noticed Kishimoto's patterns in writing. If it was that obvious, SS would be end game by now, but it isn't and it has only been disproven more with Sakura not trusting Sasuke and all the NS scenes we got in the past 50 chapters.
This is what we do know:
-According to 573, she said she will be together Naruto to the end. She said this after 540, after the confession, after everything that has happened and knowing full well that Naruto and Sasuke are going to fight. She has chosen Naruto to be the side she wants to be on.
-She doesn't have good feelings about Sasuke and is very negative about his existence.
-Sakura's confession to Naruto was real and Kishimoto confirms this. Again, nothing will change this fact.
-Sakura was sort of bashful and embarrassed around Minato when he made the girlfriend comment. She didn't say no, she just wanted Naruto to shut up. Funny enough, parents getting involved in your love affair is embarrassing.
Besides, look at this:

Does this really look like she is thinking "Wow, Sasuke is the greatest guy I have in my life?"
I don't know about anyone else here, but if there was someone I love who I think is a "great person," I wouldn't be imagining them surrounded by fire and covered in darkness with a depressed look on my face feeling like I was ashamed of it.