It would be rather difficult to miss the fact that alchemy, and the alchemy in FMA is loosely based on the Kabbalah. I mean, the fact that the table of the Spheres appears on the Gate is a dead giveaway...
This is rather interesting, seeing the contrast between this and the Seven Deadly Sins, which are a Christian idea. In a sense, the two really have nothing to do with each other.
But since we have seven Sins and I have nothing better to do with my time, I decided to take a look at the Seven Spheres and see how the FMA characters fit into those.
I would also like to say that I am no great Kabbalist, and my understanding of this stuff is sort of limited, but I'm working here with what I know (besides the fact that I don't think it needs to be analyzed too deeply here. This is just a manga/anime series; let's keep things in proportion!)
The first Sphere is Hessed, 'Misericordia', or Charity (loosely). The obvious choice for this would be Alphonse. He's a very forgiving character, unduplicitous, and infinitely caring, though I believe he's more like that in the anime than in the manga. We also see how forgiving he is towards Ed, and doesn't blame him a bit for what happened.
The second is Gevurah, 'Fortitudo', or Courage/Strength. That would be Edward. Do I even need to explain why? Throughout the series we see his courage, his determination to win despite the odds. His strength is expressed both physically (ex: he's a good fighter) and mentally (ex: his refusal to make noise during the automail surgery). Gevurah is also connected to Judgment, and we can see how judgemental Ed can be. He doesn't cut himself any slack, and doesn't cut other people much either (look at what he said to Rose at the very beginning of the series: keep walking, you have two good legs to stand on). When he sees injustice or evil, he tries to deal with it (Lior, Magwar, Youswell...).
Number three is Tiferet, 'Gloria', or Glory. It's also connected to Mercy. Tiferet is somewhere in between Chessed and Gevurah, and also is connected to the other spheres. I considered Winry, because she does complete the triangle with Ed and Al. Another option might be Izumi, considering that she (in a sense) pushed along the human transmutation that started the whole mess by teaching them alchemy in the first place, and later on you have the part where she forgives Ed and Al for what they did. Also, in her you see the combination of retribution (in her violence toward them) and mercy (in how she cares for them). You can say the same thing about Winry too, actually, if you look at her bouts of violence toward Ed, but how it reflects her worry about the brothers.
The fourth is Netzach, 'Aeternitas Victoria', or Eternity. I debated about this one for a while. One choice would be Ling, with his searching for immortality. The events of chapter 54 could either support this (since he got it, sorta) or not (since it looks like he's on the wrong side now). I suppose we'd have to wait and see. Another option might be Hohenheim, considering his body, though we run into the same problem- we're still pretty uninformed about what's going on with him. 'Father' is not really an option, since he's on the wrong side and I'm trying to look at the 'good guys' here.
The fifth sphere is Hod, 'Decus', which I'm really not quite sure how to translate. Hod is sort of paired with Netzach, which means it could be Ran Fan (in the Ling option for Netzach), or Trisha (in the Hohenheim option). I don't really understand this sphere at all, so it's hard for me to speculate about.
Number six is Yessod, 'Fundamentum', which I would also translate as Source. This sphere is the one connected to family, which leads me to choose Hughes for the position. Hughes' family was one of the few unbroken families in the series (until you-know-what-happened). He was also a sort-of father figure to the Elrics at some point, and urged Roy to get married. So if we decide to read into this ridiculously deeply, we can ponder whether his death symbolizes a lack of continuety. This could possibly be strengthened by what happened to Ling, if we go with him for Netzach (eternity), since you could say that the eternity was cut off too. On the other hand, if Ling is eternity you could say that maybe this means that he will manage to stay himself somehow, despite what happened to him. (I'm starting to get BS-ey here. Oooh, what fun!)
Last but not least is Malchut, 'Regnum', or Kingdom. That screams Roy all over. Roy wants to be Fuhrer, to lead the military and the country into a better time. Ed looks to the people immediately connected to him, or the ones he runs into, and fights for them. Roy is the one looking towards the future, and the fates of everybody. The Kingdom is at the base, because that can only happen when the others have done their jobs. Or, once it's secure, it will make everybody else's work much easier, because they will have so much more support. In a sense it's something that doesn't always seem like it's an immediate concern, but it's always lurking in the sidelines.
I guess it'll be interesting to see how the series develops and if anything I wrote here makes sense, or if (like I think I am) I'm just shooting in the dark.
If I'm already on the topic, the whole 'mind, body, and soul' thing also comes from there. There are several levels of life: Inanimate, Plant, Non-speaking Animate, and Speaking Animate (meaning, organisms with capability of speech. I need to work on my translation abilities). If something can speak, that means it has free will, which means the homunculi really have no excuse for how they're behaving.
There is also a distinction of Ruach, Nefesh and Neshama, meaning 'breath', 'spirit', and 'soul'. The breath is the physical act of living, the spirit is like the mind, and the soul is, well, the soul. Since the homunculi don't seem to have a soul of their own (in the manga. In the anime they have no souls period), all their claims about being superior beings are a load of nonsense. They're actually inferior to humans, who have souls. They're a lower form of life. That's why I found 'Father''s claims about superiority rather strange. Maybe they have an inferiority complex, so they go around killing in order to prove they're better.