I think Luffy is going to have an advantage over Big Mam in his fight with her (like he did against Enel, croc,Oz). It would make the most sense to me, as its a nice balance between the "Luffy shouldn't be stronger than a Yonkou" and the "Luffy shouldn't lose again" power-level arguments, and it fits better in the story. We still have Kaidou to deal with after all, and assuming Luffy fights him as well it would be better if Luffy beat Big Mam with an advantage so Kaidou can be the "can you really 1v1 a Yonkou on even terms" fight. There'd even be room in between for a Marine/Admiral arc as well.
As for what the Advantage should be I dont think it should be a Df advantage becuse thats been overdone(Enel,Croc).I howver wouldnt mind if Luffy won becuse he has help adavantage-just like he did aginst Oars.Maybe help from Zoro and Sanji.How cool would it be seeing the Monster Trio take on a Yonkou.
Having the Monster trio instead of Luffy take down a Yonkou is a nce way to keep the power structure intact.By the time Luffy faces Kaidou he would have powered up to take a Yonkou singlehandedly
My idea of how the battle would go is this
Bigi Mam duels the Trio fiercely for a day straight,Sanji is defeated after a day but Zoro and Luffy keep fighting.Zoro sacrifices himself to deal a great injury to Big Mam and he is defeated.Luffy alone is left standing against the Monster that is Big Mam but he defeats her thanks to her being damaged by the combined attacks of the trio earlier