She is coming to get Luffys soul.
Jk but both of them will have a role in the final saga.
Hell remember King Elizabello's King's Punch ?
What I imagined Big Mom's ultimate technique was not something with her Homies, at least not directly, but another Elbaf's inspired technique named
Mjöllnir, yes with it being named after Thor's hammer, with her concentrating all of her physical strength, combined with her armament and conqueror haki into a mega-powerful punch so strong that it produces an extremely powerful shockwave that devastates everything in its path much like King's Punch but in far more powerful with it being able of devastating entire islands and even of overpowered the Gura Gura no mi shockwaves.
But surely that was asking too much of Oda for a female character to do something that powerful and impressive.