As can be seen here, each of the Vinsmoke boys have unique powers that their lineage factors grant them. Ichiji has his lasers, Niji has electricity, and Yonji specializes in super strength.
But what is the unique power Sanji has? I doubt its his invisibility speed, as Niji showed that. Furthermore, given how Sanji still has yellow hair, I don't think he's fully unlocked his Vinsmoke powers.
imo, Stealth Black's genetic powers allow him to manipulate his hormones like Ivankov, allowing him to change genders. I think this makes sense as a power for Stealth Black as given how stealth black's specialty is stealth, being able to change appearance would be extremely useful. It isn't merely gender that hormones control. Stealth black could become taller, fatter, more muscular, or skinnier by controlling his hormones. All of which would allow stealth black to be a master of disguise, true to his name.
As for Invisibility? Given how the raid suit already granted that power, I don't see why judge would make it Sanji's genetic power.
Anyways, I think it makes sense for Sanji to end up with Ivankov's powers, Iva did train Sanji for 2 years.
I think this power would be extremely useful in boosting Sanji to the top tiers. He could use growth hormone to become larger, boosting his strength. He could use adrenaline to allow him to fight for longer and also probably boost speed.
Not to mention, I think Sanji turning into a woman would be a hilarious gag.
