Here is my Full Take on this:
Luffy have Adv. Haoshoku Haki + Flames
Zoro's Power Up was Adv. Haoshoku Haki that seem to be even Greater than Luffy's - King/God Power
So Sanji's Power Up will be Flames that will be even Greater than Luffy's (Black Flames or many Colors as hinted by Splatoon) - Sun Power
After that, Luffy's Final Power-Up will be Combination of Zoro's Power & Sanji's Power
Meaning Adv. CoC + Flames which results in King/Godly Power + Sun Power -----> Leading to Sun God Luffy
In Conclusion:
Zoro's Power-Up = Overwhelming CoC
Sanji's Power-Up = Overwhelming Flames
Luffy's Power-Up = Sun God Form = Overwhelming CoC Flames
This is also Hinted by Three Calamities, Jack (Water) - Queen (Land) - King (Sky)
Jack was Defeated by Inurashi Moon Transformation
Queen will be Defeated by Powerful Flames (Rainbow Flames which refers to Sun + Okama + Germa + Splatoon + Emotions)
King will be Defeated by Powerful CoC
So to Defeat someone who represent Sea + Land + Sky who is Kaido, you need all Those Three Power-Ups Combined
In other words: New Transformation + CoC + Flames ---> Obviously Sun God
They are also like Opposite of Inurashi, Zoro & Sanji's Power Ups in their Nature:
Inurashi Transformation is related to Moon ---> Luffy's Transformation will be related to Sun
Zoro have Demonic CoC ---> Luffy have Divine CoC
Sanji will have Rainbow Flames ---> Luffy will have Pure Sun Flames
However it will be a Temporary Transformation (Similar to Nightmare Luffy)
How will it happen?
Well, Nightmare Luffy was Luffy empowered by Shadows, so this time we have Big Mom here who can bring out Luffy's Full Potential, in other words, Luffy gonna absorb BM's Souls which results in him becoming Sun God
Meaning that Kid & Law's Fight will somehow lead to BM's Soul getting into Luffy
Could it be that Luffy will lose again against Kaido & could be dying, but then Law after BM's is Half-Defeated & her Soul leaves her Body, will capture that Soul using his DF & then give it to Luffy