It is still unclear whether "Fairy Tail" chapter 503 will release this week or next week. However, rumors are rife that the next chapter will focus on Natsu and the real relationship between Eileen and Erza.
There are two theories about the implication of Natsu's tumor, according to Reddit. While some fans believe that the tumor is a sign of Natsu transforming into Ethereal Natsu Dragneel (END), others hold the belief that he is going through the process of dragonification.
Some avid fans believe that it is likely for Natsu to tap into his inner END in "Fairy Tail" chapter 503, something that will be triggered by the torture he received from Dimaria. In case Natsu will really connect with his END, it is speculated that Dimaria will be the first to experience the supreme power of Natsu's END.
On the contrary, some fans believe that the tumor may be a sign that Natsu will change into a dragon through the process of dragonification because he has overused his Dragon Slayer Magic. Apart from revealing the meaning of Natsu's tumor, it is reported that the forthcoming "Fairy Tail" chapter 503 will also focus on the relationship between Eileen and Erza.
In the previous chapter, Eileen tried to get the Fairy Heart from Mavis, who only tricked the former using her illusion powers. Finding out the she has been tricked; speculations are now rife that Eileen will hunt down Mavis. In the process, Eileen will come face-to-face with Erza, who is believed to be either her mother or sister.
In "Fairy Tail" chapter 502, Zeref ordered Eileen to go after Mavis, something that she readily agreed to so that she could punish the latter for making her look like a fool, according to Manga Stream. "Fairy Tail" chapter 503 spoilers have it that Eileen could lose her cool in the rage and could easily choose to forget the rest of instructions from Zeref, and eventually not wounding Mavis. Here is footage for further information on the upcoming chapter: