[This may not be 100% what I'm thinking or accurate, I'm going off of what I remember.]
7 Half Bloods shall answer the call;
To storm or fire the world must fall;
An Oath to keep with a final breath;
And foes bear arms to the Doors of Death.
7 half bloods shall answer the call - our 7 protagonists: Percy, Annabeth, Piper, Jason, Leo, Hazel, Frank.
To storm or fire the world must fall - In the book The House of Hades, Jason encounters the South Wind, if I remember correctly - he has two formes, when Greek and Roman. Jason has to make his choice: storm or fire? Greece or Rome? In the end, he chooses fire, Greece, in which the god says, as Jason is leaving, something along the lines of, "When the choice of storm or fire comes, just remember me," thus, solidifying my belief in that. As for the world falling... Well, Gaea IS the world. Gaea will fall to Greece or Rome.
An oath to keep with a final breath - One of two possibilities; Leo goes back to find Calypso, in which he well... dies. OR, the more realistic possibility, Nico Di Angelo dies whilst helping Reyna get the Athena statue back to Camp Half-Blood and to make peace with the two camps.
And Foes bear arms to the Doors of Death - Bob, Damasen, Percy and Annabeth fighting for their lives in Tartarus.
Hoping you all agree with me, and I hope you all enjoyed reading this.