Ok, keep in mind that these are NOT polished. They aren't what I believe will happen. I'm just making random theories that might happen, according to the research I have done.
#1: Father is Riza's Father.
Okay, as sempai has pointed out to me, the last thing we need in FMA is another random family relation. (i.e. Envy was Ed's brother via the anime) Nonetheless, I feel this might actually work.
Father and Riza's mother (General Grumman's Daughter) had Riza. When she was older, Father marked Riza with the tattoo as a sacrifice. He then killed her mother (for some reason...) so Riza decided to run away and join the military. (Either just to run away or in hopes of dying.) Riza may or may not know what the tattoo stands for. The Homs. also don't know who she is, else they wouldn't have tried to attack her.
Sub theory 1:
Father needs Riza as a key (fire's key?) to open one section of the gate. A large amount of fire is needed so Roy will have to be used to make everything work. (Not that I believe Roy would willing do anything that would harm Riza. Theory needs more work.)
Sub theory 2:
(Don't kill me) The 'groom', who is Father, needs to have sex with the 'Bride', who is Riza, in order to give birth to a pure, untainted child, a.k.a. the P. Stone. (o-O) Thus once Father realizes who Riza is, he will order the homs. to bring her to him. (This theory creeped out almost everyone in the group. Most said that it was too creepy to actually be a real plotline. Jon said that he read something before where a father raped his daughter in order to open the gates of hell using her uterus. Jon's reading habits worry me.)
#2: Riza has a twin brother.
Oh the corniness, but give me a chance on this one. Riza has a twin brother who also has a nearly identical tattoo on his back. The difference is that the array would be different (water?) and there would be a moon instead on a sun. Both are needed as keys to unlocking the stone.
Man what is with alchemy and sex. Another part of the hermaphrodite research states that the brother and sister (refering to the two parts of the hermaphrodite' have to have sex to make the stone. (Also creepy, the thought of her brother coming to visit and he starts hitting on her. *shivers* Note to self: More research on the brother/sister part of the article.)
#3: Riza is/represents the hermaphrodite.
A lot of the symbols on Riza's back link up to the alchemic reasoning of the Hermaphrodite child of the sun and the moon. Such things that are included are the dragons falling to earth, the cross, the sun, the salamander, and fire. (If the dragons are seen as serpents, then it also supports this theory.)
Sub theory 1:
Riza is a hermaphrodite. (o-O)
Sub theory 2:
Riza's personality represents that of the hermaphrodite. Meaning that while she is a woman, she is also have masculine personality traits. (Then again, so does Winry sometimes. NTS: Look into more.)
Volume 6 Manga
Noticed a strange image in Vol. 6, when Ed is inside the gate. Next to the image of a lion eating the sun, there's a picture of an angel(?) with eyes on her wings. Must look into that.
Animal Symbolism in Alchemy
Blackening - Black Crow, Raven, Toad, Massa Confusa.
Whitening - White Swan, White Eagle, skeleton.
Greening - Green Lion.
Rapid cycling through iridescent colours - Peacock's Tail.
White Stone - Unicorn.
Reddening - Pelican feeding young with its own blood, cockerel.
Final transmutation - Phoenix reborn from the fire
'Another symbol of this stage was the dragon, a familiar inhabitant of the alchemists flasks. The dragon is however a more complex symbol and is also used when winged as a symbol for the spiritualising of the earthly substance. Thus to the alchemists the dragon appeared at the beginning and at the end of the work.'
More on Dragons
The dragon in flames is a symbol of fire and Calcination. Several dragons fighting is symbolic of Putrefaction. Dragons with wings represent the volatile principle; dragons without wings represent the fixed principle. A dragons biting its own tale is the Ouroboros and signifies the fundamental unity of all things.
Def of calcination (process of being calcined)
More of Serpents
Two serpents represent the opposing masculine and feminine energies of the Work. Three serpents stand for the three higher principles of Sulfur, Mercury, and Salt. Winged serpents represent volatile substances; wingless serpents represent fixed substances. A crucified serpent represents the fixation of the volatile.
(Question: Are they Dragons or Serpents?)
More on Crowns
The crown symbolizes the successful completion of an alchemical operation or the achievement of a magisterium. It also signifies chemical royalty or the perfection of a metal.
(Def of magisterium)
More on Fire
"That which the fire operates is alchemy, whether in the kitchen or in the universe" as stated by the alchemical forefather Paracelsus 1493-1541. More specifically this ancient alchemy symbol represents divine energy, purification, revelation, and transformation. As such, fire was (and still is) an important sign in the alchemist's arsenal of emblems.
More on the Sun
The ancient alchemy symbol of gold. Worshipped in many cultures, specifically Egypt where Amon-Ra was personified by the evening sun setting at the end of the creative process. Alchemists understood the process of creation, and utilized the symbol of the sun to focus on creating desired effects in their practice – both physically and philosophically.
(NTS: The“(Al)Chemical Wedding” of Christian Rosenkreutz, and the Archetypal Sevenfold Pattern of Transformation)
Volume 8, Chapter 30, Page 24-25
Looking at this picture, you'll notice that, dead center, there is a picture an 'Caduceus' which is two serpents twining around a rod. (This symbol is often used which medicine and healing.)
"The staff with the twining snakes is based on the staff of Aesculapius, the god of medicine. This was a simple staff with a single snake entwined around it. It has, with variations, become the symbol of medicine. The use of two snakes symbolises the duality of everything, including health. The two snakes cross 7 times (including the heads and tails), symbolising the 7 chakras or main 'energy' centres of the human body as described in Eastern medicine. The twining snakes may also be seen as representing the double helix of DNA." (FN:1)
Volume 12, Chapter 53, Page 17-18
On this page, Edward explains the array from the Xerxes Ruins. (Has there been a comfirmed name for this yet?) He explains the the dragons being upside down represent 'God being thrown to earth by human hands'. He goes on to say that the twining male and female dragons represent Hermaphroditism is the representation of alchemy that indicates a "perfect existence"
*work work work work work* Feel free to read, though I can't promise your sanity to stay intact. *work work work work work*