The explosive Fairy Tail chapter 504 "Rift" will bring about the climax of the Alvarez Empire Act as both sides could reveal their "Trump Cards."
Chapter 505, entitled "Trump Card" could refer to different secret weapons from both sides.
An easy assumption is Larcade Dragneel whom Zeref has previously called his "secret weapon" in Chapter 494.
In the Fairy Tail Subreddit, prediction by user 'GooseRider960' states that Natsu will simply divert from the face-off with Gray and proceed to decimate the Alvarez Empire army. His rampage to attack Zeref will be halted, though.
"Larcade stands before the guild, and senses an intense heat. He lifts his hand up just in time to catch E.N.D's punch, calm smile resting on his face, before swinging him into a rock," the user predicted.
Most fans believe that the Gray vs Natsu fight will not take place this early and their meeting is only a teaser. However, there are those who believe that setting up Gray and Natsu as the climactic battle would be too similar to the Naruto-Sasuke battle in Naruto Shipuuden.
The prediction from user SirSamuel016 takes a different direction with the fight extending throughout the chapter.
"Meanwhile, Natsu and Gray still exchanging blows, Gray's body almost completely covered in black markings now," the user wrote.
In this prediction, Gray is filled with hatred towards END and sets aside the fact that it's also his friend Natsu he's attacking. On the other hand, Natsu tries to hold back but eventually gives in.
In this scenario, the "trump card" refers to Gray's Devil Slaying Magic-a secret spell that he learned through a book that he acquired in his time with Avatar. Gray has repeatedly referred to this in previous chapters. The nature of this magic may be revealed in the next chapter.
These are two interesting predictions that both center on Natsu, but "Trump Card" may not even refer to him or Gray at all. There are other battles and as readers from Mangahelpers speculate, it could refer to "Mavis secret plan to defeat Zeref."
In chapter 504 "Rift" Mavis emphasized her skill as a "tactician." Throughout the arc, most fans have criticized Mavis game plan but she has eventually emerged to escape the clutches of the Spriggan 12 and rally her comrades in the battlefield.
Perhaps we have underestimated The First and Trump Card may refer to something up her sleeve.