With Whole Cake Island arc coming to a close, I realised that we probably won’t see Luffy fight/defeat Big Mom before the series ends.
My theory hinges upon assumption that one Yonko gets one big arc. This is the Big Mom’s arc, so we move on to the next Yonko arc: Kaido’s arc.
Big Mom’s arc is almost completed and Luffy defeated a 1 billion berries opponent. This arc establishes that Luffy’s power level is about 70-80% of Yonko level (setting himself up nicely vs Kaido next arc). We probably won’t see another drawn out arc with Big Mom pirates again. So we won’t see Luffy fight Big Mom.
We come to Kaido. Luffy will defeat his first Yonko with help from Law, Drake, Kid, Hawkins, Apoo.
After this arc, Luffy will encounter Shanks to return the hat as Luffy will be considerate a great pirate. Of course there will be no fight between Luffy and Shanks.
Then we come to Blackbeard. I believe Oda is saving this fight for the series finale just before the final war between Straw Hat Pirates + Allies + Revolutionary Armyvs The World Government.
*Theory by smao815