I feel like Sanjis power will be based on his heritage.
His first power was a fighting style given to him by his adopted father
His second power was fire given to him by his mothers sacrifice
His third power was a body forced upon him by his birth father
If Sanji continues down this path I believe the next power will come at the cost of the exoskeleton where he’ll gain a more lunarian body more related to Soras heritage.
If Sanji gets CoC (which I feel he will) it’ll probably merely be for observation killing tho.
He's not going to have any more scientific power-ups, he never wanted any and it's something that's been imposed on him. Anyone talking about germa or vegapunk power-ups is delusional.
His next power up will focus on his flames and haki, so either oda reveals that his current flames are just a small part of his true power, which is likely given that he's been teasing sanji's firepower in the last few sbs, or he gets a haki boost to support hotter flames, or just increases the AP/DC of his attacks.