The following is not an extrapolation about what will happen in 51. There will be spoilers, no doubt, but nothing like "I think that Al this or Ed will that". <3
This is inspired by a comment I left and then was responded to. So, I think it deserves some elaboration. Forgive me if I become incoherent.
There are heavy influences of the Renaissance in FMA. From the Baroque (I believe it's Baroque) music, the Gate, the Seven Deadly Sins and many other examples, we know that the knowledge base of the world of FMA is based out of the Renaissance ideas. Alchemy and Alchemical understanding comes to us from this time of experiementation and growth.
This is what seems obvious (to me, anyway). FMA is an allegory. It is making a commentary about the means in which people destroy each other. Some are physical, example being Ed and Al, some are emotional, example being the homunculi, and some are other worldly, example being the alchemical concepts themselves. The Universe in which Ed and Al have grown up consists of the idea of one-for-one. It's given the title "concept of equivalent trade" but it essentially, to give you must get. The world works on a barder system where no one can escape the system. The only way to escape the system, at least conceptually, is to obtain a Philosopher's Stone. It's a free pass from mutuality.
The heavy Christian imagery and characters, Rose, the Homunculi, et al, is not atypical from the adult anime I've seen. NeonGenesis Evangelion is all about reconnection to "Lilith" the first woman God created from the dirt who refused to obey God and submit to her husband. She was then expelled from the Garden of Eden. She represents the equality and understanding amongst people in that anime. This is just an example.
The use of Dante for the controller of the Seven Deadly Sins is important in and of itself because Dante was the one (Dante Allegri that is) who said that certain sins would put you into more and more severe circles of hell. The deepest being betrayl. So, the gate which Dante's soul has passed through so many times to get affixed to his/her (hereby referred to as hir because we do not know Dante's true gender) that ze would be in almost of control of the gate. The gate, which has been pointed out, that looks like gate which was drawn based on Dante's inferno.
The Seven (main) Homunculi themselves are based in what they are conceptually. Gluttony, a fat child-like man. Lust as a beautiful woman, whose beauty kills (her nails). Sloth, who only moves quickly when water (water, of course, having the property of not appearing to move but being on constant motion). Envy can shape shift, as you always want the person's attributes when envious. They all have this metaphorical manifestation in their killing power.
The timeline for FMA, as I've come to understand it, is 1910's due to the AU action late in the series. Therefore, it can be assumed that 400 years prior is the late 1400s early 1500s. During this time, Christianity was going through a major overhaul. The Babylonian Captivity (when France literally arrested the Pope and took him to France) threatened to destroy Christianity. Several Popes claimed power and corruption enetered the Church. This time period gave the set up for Martin Luther later in the 1500s. Also during this time (or slightly before as it was the late medieval period when this happened, but I digress), the switch to the Gregorian Calendar from the Julian Calendar was made.
The Gregorian Calendar is the calendar used internationally now. This calendar switch, in an AU if the Catholic Church had crumbled to nothing, would be the 'old calendar of the old religion' referred to by Dante earlier in the series. The different patterns in Cathedral architexture (especially the windows, as we've seen) makes circular patterns which do resemble the Alchemical Transmutation Circles we've seen throughout the series.
Getting back to the greater Christian contexts of FMA, the first example we see, in the first 2-3 odd episode (I'm bad with keeping the numbers down, please forgive me for ranges), is that of Fundamentalism and Fanatacism as exercised by Lior. It destroyed the city completely and when new, contrary ideas come into play (much like the Native Americans and the European settlers), Europe reacts by killing them for something greater- God- as represented in the Philsopher's Stone.
To come in contact with the Philosopher's Stone means that you see 'The Truth', this idea comes to us after the soldiers have their episode in Lior and Al has his transformation. Al sees the truth and it becomes one with him. He is not punished because he is pure in his ideas and actions. Scar is not pure because he wants only to exact revenge and therefor dies.
To create the Philosopher's Stone, you must kill many souls. In Christian thought even up to this day, those who are considered 'closest to God' are those who are socially secluded and devote themselves to prayer and meditation. They must kill the society inside themselves and release their soul (or kill the soul's connection to others) for it to be made.
The idea of the Philospher's Stone being God also ties into the idea that the Elric brothers want to come into posession of the Philosopher's Stone only to right their wrongs. They do not want to control, they merely want to repent. The theme of Redemption and repentance. Al and Ed repent for their wrongs (trying to do something only the Philosopher's Stone can do) of trying to resurrect their mother. The Alchemical Law of Equivalent Trade (not unlike a commandment.. a law that must be obeyed as given by the men who came before) punished them by taking Al's body and then when Ed renegged the acts of God again with affixing Al's soul to the armor, Ed lost his arm and leg. They are repentant about the wrongs they have done and wish to be redeemed, that redemption takes the physical form of their bodies' respective restorations.
There is a strong idea that this world is hell. You pass through the gates and find this world with all its horrors and shortcomings. They are immediately exposed because Ed is not from this world. If the world had been the other way, where the AU were flipped, it would be just as horrific. Without deviating too much, I want to point at that maybe they're not hells for each other (though there is no other comparable word) but more Foils.
Since I'm starting to drift, I'm going to end this here. I hope I haven't confused any of you too much. <3