Sanji's PU is not completed imo. Since he upgraded his Haki(attained Hardening CoC coating), logically he should also upgrade his flames since he should be able to handle hotter flames now. There is a reason Sanji still yet to use a named attack after his PU. My money is on either purple or pink flames for a new Jambe mode, let's call it Sutur/Lucifer Jambe for now.
Also think he's gonna get a flame upgrade, just not quite sure it'd be happening this arc.
Mostly coz it doesn't quite feel necessary yet. Sanji usually upgrades his flames when he requires more firepower but technically he's already acquired that with his CoC awakening, so I think Oda might maybe want to focus on that a bit more first.
That said, if we are going to be getting wings v Imu or wings vs Evil Nika then maybe he upgrades his flame but even then I can't help but feel like Oda's gonna be putting more emphasis on the defensive side with him this arc and have Zoro show out on the offensive.
If Oda gives Sanji an offensive PU in addition to a defensive one other than his CoC coating like black flames for example, I can't help but feel that it'd take away a bit from Zoro who's being propped up as Nika's sword.