With Sanji, there are two paths I can see for him going forward:
-His powers gradually fade and he reverts back to normal soon enough.
-Sanji goes all in on his powers out of desperation, becoming fully modified, but losing his personality in the process. At that point, someone will need to fix him.
In any case, I could see this becoming a risk and reward power-up for Sanji in the future, that he can tap into if cornered. Sorta akin to Monster Chopper pre-timeskip, where it becomes a last resort.
@Watch-man:
Thats your guess, as much as I choose to see it as permanent change. Lets wait and see.
Oda sorta has problems with sticking to permanent physical changes in his characters if he had been used to them for a long time. I still remember how Luffy's chest scar was always appearing and disappearing a bit after the timeskip because Luffy never had a scar before timeskip. And then you have stuff like Luffy losing a teeth through a chunk of WCI to make Sanji's attacks actually look hurtful, only for him to recover it later in a throaway scene.
Plus, stuff like merchandise incentivizes characters to not change, thanks to reference documents and the like.
The only changes that have actually stuck through the story were Zoro's scar and Nami's tattoo, but those were very early cases where Oda was still experimenting with the East Blue. Zoro never had been seen shirtless before facing Mihawk, so it was easy to get used to it. And Nami only revealed her Arlong tattoo during the AP arc, so it was a design choice that lasted less than an arc.
But Sanji? Oda has been drawing him like that for over a decade now. I really doubt he will deviate from that.