I won't be responsible for ruining your expectations of the game and such. So, don't blame me!!
**************SPOILERS START HERE**********
Still here? Be my guest then. In this entry I'll post some reasons as to why I think that Fang and Light are not only very much alike but made for each other (or FLight theory [yeah, I just made that up right now]). So please, no bashing or anything of the such here. If you want to comment, feel free to do so.
Let's start, shall we?
Well. Let's start with their personalities.
As many of you that have played the game, read the novels, listened to the cd-dramas and support the FLight fandom, know that Light doesn't show her emotions that easily but she's not heart colded. When she's trying to protect the ones she loves, she tries to make the impossible, possible. Fang is more easy going, true, but you can see her despair at some points due to her lack of power to protect the ones she loves, and also goes to the extreme to protect them.
You can see this in several moments in the games. Let's talk about Light first.
Yeah, is true that Light didn't believe Serah when she told her she was a l'Cie. However when that changed she practically gave up everything to chase after her in order to protect her and save her. She decided to purge herself and defy a whole system, and let's not forget that a 'god' too just to protect her only remaining family. Then after she requested her assistance to protect the future and caused her sister's death she decided to become crystal with the hope to one day wake in the same era as her sister and meet up again and be happy. Sad thing is that didn't happen, and the 'all mighty' (and a$$) Bhunivelze woke her up with the divine task to save souls. Ah! but that was not just a request and simple task. Bhunivelze practically blackmailed Light. He would make sure to have her in his grasp to do this by having Serah's soul hostage (or that what he told her), and that Serah's soul would be "reborn" in the new world if Lightning performed the task asked of her. Of course Light being Light, didn't hesitate. If that's what it took to have Serah by her side again, she'll become a puppet of the 'gods' one more time (though, she was stronger than Bhunivelze expected and Light didn't quite trust him in this and Light once more stood up against a god to protect what she loved the most, her family and the future they could live in).
And now, let's go to Fang. Just as Light, she goes to extremes just to protect the ones she loves.
As a child she didn't like the fal'Cie, not even the 'patron' of her village (which was Anima) or even Etro. But she turned l'Cie in order to protect Vanille, since she knew that she didn't have the power to fight the vipers of Cocoon. She knew what the price of it would be, and she did it for her family. During her journey, as everyone decided to go against their fate, she turned her back on them, just so she wouldn't have to see how they became Cieth in case they failed. She along with Vanille, sacrificed herself and became Ragnarok to save Cocoon and her new family. Maybe she could have done it with someone else, but Vanille felt the same as her. Many centuries passed and she was freed from crystal and awakened in the new world. During the years they were awake, she knew that Vanille was being used because of her new power, and she left her alone (this reminds me a lot when she left her too in the first game in the power plant) in order to protect her. She went to the Dead Dunes to find a relic that Vanille needed to perform a ritual and help the souls of the dead. Fang knew that if she gave the relic to Vanille, it'd mean her death for sure, so she left in order to get that relic but NOT to hand it to Vanille or anyone so they wouldn't use it since it had the power of a 'god'.
As you see, both Light and Fang do almost anything to protect their family. Which is what makes them so similar. Also, let's not forget that even Serah said so. When she meet Fang for the first time in the dream world, she said Fang reminded her a bit of Light.
Probably some of you might say that this just proves that there's Farroncest and Fanille. But I think otherwise.
Let's go now to the situations I think make FLight more possible. (I'm taking aspects of the Japanese and English version.)
Lightning only has allowed two people in the game to call her 'Light'. One of them is Hope, and it was since she became like a guardian figure to him. And the other one is FANG. This isn't show that much in the English version, but it's there. Even if Lightning did spend time with Sazh and Vanille, they aren't allowed to call her that. In their reunion scenes in LR, the only one who tells to call her Light is once again Fang, and gives her a tiny smile.
Light left on her journey at the end of FF XIII to save Vanille and Fang, even if Fang told her not to go look for a way to free them. But I believe she did it more for Fang. Of course she couldn't leave Vanille there, but she knew that Hope would be more glad to see Vanille again. So that's why she decided to go on her journey.
Fang has always been more stable and able to complete her goals with Light by her side, and also to have hope and stand up again (also I read a fic that make me ponder this and see how true it was). Before Light appeared in the picture, Fang was unable to find Vanille after they got apart in the Plant. But when Light appeared, the rescue was a success. In the fifth Ark, it was Light the one who asked Fang for her strength and help to fight together (and let's not forget she gave her her hand to take, I don't think she even did that with Hope. And the hug doesn't count!). She stayed behind with her before they headed to Oerba. In the fight with Orphan, after the illusions of them being transformed in Cieth, Light was the one who had Fang's lance and gave it back to her. And they nodded at each other. Fang said that it was the second time she saved her. In LR, when they meet, Fang greets her and places an arm on Light shoulder and says she was just like at that time (referring to their l'Cie days). Also, Fang left Vanille to get a relic, but never came back. Fang couldn't get the relic UNTIL Light appeared, being the Savior and all she had the power to do it. Sadly, the relic was taken by the group that is protecting Vanille and Fang got angry 'cause she knew it meant Vanille's death undoubtedly. Light tells her that Vanille will die for sure, that she can't save her as Fang wants, but only she can make Vanille react and bring her back. While she says all this, she has a hand on Fang's shoulder, and tries to comfort her that way. And before she turns out to leave, she gives Fang a small smile. Fang is more relaxed now and says she knows that already.
This again shows how much they understand each other. And how Light is 'more touchy' with Fang than with anyone else, not including Serah. I've never seen Light do that with someone else besides Fang (or Hope, but we've made clear that she's more like a guardian to him).
Some of you may think that Vanille is Fang's love interest as how Fang react to her. I'll be honest, and some of the dialogues can lead to think that, but there can also be different interpretation of it. For example: When Fang says she'll tear down the sky for Vanille. Well, in the Japanese version she doesn't say something as drastic. She says that as long as Vanille is fine, she doesn't care what happens.
In LR, Fang says that they've been together (Vanille and she) since the beginning, so they'll be too at the end.
Then, while Vanille is performing the ritual with the relic, Fang is behind her, holding her and saying: that even if she dies, she won't let go.
As I see it, Fang reacts basically the same way as Light when it comes to someone dearly to them. In this case, sisterly love. In Light's DLC, you can see her holding Serah in a very loving way, and even touch her cheek gently. She turned crystal for her, and even became a puppet just to be with her one more time. So to me, those acts show love, yes, but love for family. Both show it in this way.
There's also this thing, that besides Serah and Snow, there's just been one person to express something for someone. And it was Hope TO VANILLE. He said he liked her smile. And I don't think it was just teasing. In a very subtle way, he's shown interest in her. Or do you not remember the ending of the first game? He's looking around when Serah and Dahj appear. He was looking for Vanille. And in LR, Light mentions it a bit. But Hope is now emotionless and kind of casts it aside, but it was there. And they were more touchy feely with each other.
I know that the producers wouldn't make gay/lesbian couples Canon. But just think about it, if Fang were a dude, I do think they'd be considered canon because of the reasons I've mentioned.
I think I've forgotten something. But I think I've made my point as to why Fang and Light are indeed a good couple, made for each other and maybe not officially canon, but closer to it from my view and perspective.
Thanks for reading. Pardon the typos, grammar and wall of text.