WARNING: The following contains spoilers for One Piece, by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English from Viz Media.
Recently in One Piece, things haven't been going all that well for Vinsmoke Sanji. Everyone's favorite cook has been receiving numerous upgrades that have made him stronger than ever -- potentially even stronger than a Yonko commander. However, in this particular situation, the stronger Sanji gets, the worse the outcome looks for him -- and his iconic eyebrows might be the biggest clue as to what his fate may be.
All of Sanji's recent power-ups come from his father Vinsmoke Judge, or more specifically, the experiments he performed on Sanji and his brothers while they were in the womb. Hoping to create the perfect super-soldiers, Judge altered his children's bodies to the point where they each gained superhuman strength exoskeletons as well as incredible healing factors. Each child also received their own unique specialized abilities, making them deadly in different ways. Unfortunately, Judge also desired his children to be merciless, as he felt that would make them even greater soldiers in his Germa 66 army; accordingly, he removed their senses of sympathy and empathy.
While none of his brothers were able to avoid this fate, thanks to his mother Vinsmoke Sora taking a dangerous drug, Sanji was spared this outcome, but at the cost of her health and eventually her life. Learning of his mother's sacrifice from his older sister Reiju, Sanji dedicated his life to living the way his mother would have wanted him to, with his kindness becoming one of his core traits in adulthood.
Unfortunately, as of late, it seems that Sanji might no longer be able to keep his promise to his late mother. According to Sanji's own hypothesis, by donning the Raid Suit given to him by family at the end of the Whole Cake Island arc, he's inadvertently activated his Germa 66 enhancements. Believing he didn't have them at all, he and everyone else was shocked to find out that they were just latent, waiting for some external source to awaken them. Now Sanji is stronger than ever, fully believing he could take out Kaido's commander Queen with ease, but seemingly at the cost of his humanity.
Aside from the lack of Germa 66 enhancements, there was always one other clear difference between Sanji and his siblings: their eyebrows. While all of his siblings have eyebrows that curl toward the left, Sanji's had always curled to the right until the latest chapter. While having an unbreakable exoskeleton and practically instantaneous healing abilities might seem like much more important changes, the change in his eyebrows is actually quite significant.
When the eyebrows of Sanji's siblings were revealed, it didn't take long to notice that they closely resembled the number six. With the siblings unsurprisingly each having two eyebrows, it looked like they were forming "66." This obviously stood for Germa 66, the name of their villainous army and kingdom of science of which they were the leaders. In many ways, those eyebrows were synonymous with Germa 66 and the evils they committed, which is why Sanji's eyebrows changing is so significant.
Sanji has spent his entire life trying to escape his Vinsmoke legacy and Germa 66 connections, with his mother literally sacrificing herself to make sure he didn't end up like the rest of his siblings. Sanji's eyebrows were in many ways a symbolic representation of that desire. If the standard Vinsmoke eyebrows represented the evil and heartless principles of Germa 66, Sanji's conversely represented an inversion of those same principles.
In the earlier chapters leading up to 1031, even though Sanji had awakened his Germa 66 powers, his eyebrows hadn't changed in any way. Interestingly though, the first time readers see that they've changed is after Sanji has been accused of striking a woman -- an act all fans know he'd never do (much to the chagrin of many in the past). Sanji even recalls what his siblings did to a servant girl after he hears the accusation, also remembering just how much he hated them for doing so.
It's quite possible that the eyebrows are meant to represent just how much Sanji is becoming like his siblings -- specifically, how merciless they are. In fact, fearing that he'll become just like them, he's even asked Roronoa Zoro to kill him if that indeed comes to pass, going as far as breaking his Raid Suit in a desperate effort to stop the changes from advancing further.
Currently, it's unclear just what's going to happen to Sanji. While it isn't likely that series creator Eiichiro Oda is planning on killing off one of his most beloved characters, it's quite clear that he has big plans for him, as well as for his ever-iconic eyebrows. With Chapter 1032 not releasing until next week, fans will just have to bear the wait until their burning questions on the subject can be answered.