So here is my prediction on the Prophecy and the rest of the series
Mark of Athena
Mark of Athena will take place from the Prospectives of Annabeth, Reyna and Nico
Mark of Athena will hopefully begin with a big reunion of Percy and Annabeth. It will be arkward between Jason, Reyna and Piper because Jason and Piper are dating and Reyna and Jason were almost dating. Another Love Triangle will form between Frank, Leo and Hazel. After the massive Reunion they set sail for Rome, with a heap of CHB people and CJ people and the seven. Meanwhile Nico shall be in his prison. Gaia has imprisoned him in earthen cage which he cannot break. He hears about Gaia's plan to awaken more giants using a special power known as the Mark of Athena.
On the ship they shall be a power strugle between Jason and Percy between who gets to lead, they might even fight eachother, though in the end I think Jason will lead. Reyna and Piper will talk it out and try to figure out what to do, though they both want Jason so they can't decide anything. Next they get to rome and meet a god there. The god said he has been in comunication with a certain hero(Nico) who is imprisoned who has told him about the Mark of Athena, a power source that could release all giants. The god also says this will decide who is worthy enough to be the seventh. The god also says that the location of the Mark of Athena is only known by Nico so you must find him first. Nico does many escape attempts while they are planning to free him.
So Annabeth, Reyna and Octavian leave on the quest. In a day they have already found Nico though they learn that the cages are bound to a giants life force. They fight the giant an the god from earlier helps to defeat him and free nico. Nico tells them that a giant is going to activate the Mark of Athena and if he does it will awaken all the Giants.
They go to the location of the Mark of Athena and Octavian attempts to take out Nico though he just stops him. Octavian reveals himself to be on the side of Gaia. He takes the Mark of Athena and attempts to activate it though it destroys him. A giant then comes in and reveals that only a true daughter of wisdom or a immortal could control Athena's Mark. So the Giant takes the Mark and uses it to take out Reyna and Nico. He says the Mark of Athena has to go to Greece the place where most of the Giants are resting. So he releases the Mark and it Burns through Rome as stated by the Prophecy in SoN. Annabeth then runs into the rest of the group(being the six from the other books) who she tells to go in to the Giants lair and defeat him. They go in there and hold of the Giant. Then Reyna wakes up and the Giant vanishes(like Porphyrion).
Now Annabeth walks alone as also stated in the prophecy. She then meets up with Rachel who will say the rest of the prophecy which states that the daughter of Athena who walks alone is the seventh. She finnally figures out a way to stop the Mark, so she does.
Everyone meets back up and it is revealed that the giant is going to Greece to revive the other Giants and then Rick will cut it off there because he always enjoys leaving us with killer clifhangers.
This line means that 7 Half Bloods shall answer a call during the battle, that is why they will be important, not because they are the only ones going to Greece, just because they will be more important in the battle.
People assume that this means that either Jason or Percy (as storm) or Leo or Frank (as fire) will destroy Gaia. For all we know the Fire or Storm could be gods, or just storm and fire and the world could be the real world not Gaia. That line is still very unclear.
This line could be many things though so far people think that Percy will have to choose between Annabeth and the Romans. I actually think this may not even be about one of the seven and may actually be about Octavian who may have made an oath to Gaia and at his final breath he will break it and save the seven.
This could mean many things though once the Seven find the doors of Death the Giants may turn to there side. Or it could just mean Greeks and Romans bare arms at the doors of death
Jason Grace - The Leader, He will be the leader of the seven as Zeus is the leader of the gods. He will probably end up with Piper in my eyes and I also that he may take down Gaia
Percy Jackson - The Hero, He will probably end up leading the main battle against the giants and will be the second in charge
Leo Valdez - The Champion, He will have a very important role to play with the fact that he is the mechanic of the ship. He will want Hazel and that will probably lead to a fight between him and Frank where he might even use his powers to kill him
Frank Zhang - The Secret-Weapon, He will also have a very important role to play as his abilities are very powerful. I think he may assist Jason in the final battle with Gaia and that is when his fire will burn out and he will die
Piper McLean - The Convincer, She will be one of the most important people in this part because of her abilities, she will probably help close the Doors of Death using her power of Charmspeak
Hazel Levesque - The Dead, She will probably have her curse removed before the end of the series by Frank. I think she will not be able to decide between Leo and Frank though when Frank dies she will just go with Leo
Annabeth Chase - The Strategist, I know there are still some people who have doubts though I think Annabeth is the seventh after reading SoN. She will be one of the main strategists in the battle and I think she will play an important role in the next book so thats why I think its hers
Reyna - I have a good feeling that she will also die before the end of the series. Why? because when she realises that Jason no longer loves her and wants Piper instead she will sacrifice her self in battle and hand her position as Praetor to Hazel
Octavian - His role will basically be the same as Luke's. He is the Traitor and will side with Gaia. In the end I think he plays a part in the third line of the prophecy because he will return to good
Nico di Angelo - I don't think he shall play much of a role after Mark of Athena. I think Gaia will have broken him while he has been imprisoned, he shall be nothing except a broken down once hero and that is why he probably won't fight
Rachel Elizabeth Dare - She will stay around and predict most of the Prophecies because it is pretty hard to get information out of Ella. She probably won't fight because she isn't a demigod
There are all of my predictions, if I have more I will update it
Now tell me your predicitons