Sanji's final power up: Mother Flame Jambe. I don't think it is impossible Sanji will merge with the flame to stop Imu from getting it
I actually don't think it's going to be the same as the mother flame. DJ was pure fire colored red/orange/yellow/white. IJ is a mix of fire and lightning, colored dark blue, light blue and white. The new Jambe(Nephilim Jambe NJ: the name just came to me, seems fitting) is going to be an amalgamation of fire+lightning+light power, colored Purple/Pink.
I don't know what would that give Sanji, moveset wise. Maybe in addition to having burning and electrifying effect, his new mode will have light speed kicks, or flying laser kicks, exploding orb like projectile, it could just be cosmetic: star chaped of smth similar. Oda could go with it however he likes.
As for Sanji's final PU, it's going to be a Dark/invisible Jambe, composed of flame+lightning+light+Plasma. Let's call it Satan Jambe SJ. And I think just like how NJ is gained through advancing CoC, SJ would be through Soul advancement.
On a related topic, I noticed Sanji's PUs follow the four types of love in Christianity(friendship, Familial, Unconditional, Romantic). Each type has its own kind of trial for Sanji:
+ DJ is gained for the sake of rescuing Robin and better protecting his friends: friendship love trial
+ IJ is gained after making peace with his past related to his blood family and what comes with it to protect his found family: Familial love trial.
+ Nephilim Jambe(NJ) would be gained after acknowledging the unconditional love his close people has for him and as a result he gained self love: unconditional love trial.
+ That leaves the romantic love trial for last. Fitting, I suppose. Here I think Nami and Pudding will play a major role in this trial, possibly even a third mock marriage plot. The first was a marriage plot through kidnapping, the second a marriage plot through blackmail, the third could be a fake or mind controle one.