How much narrative setup DOES she have in Elbaph? That's a good question.
Is it none or some or a lot?
Are you expecting her to show up and marry Loki or something?
1. Jorul's dishonorable death and Jarul's resentment over it
2. Goldberg's Shield, Nash, who is implied to be his Dad's Soul stuck in a shield because of Big Mom
3. Loki/Lola backstory, and how Loki/Linlin met
4. Linlin's history with Elbaph after Mother Carmel Incident
5. The Giants know Big Mom ate Mother Carmel and the other kids, and the Straw Hats do not know this
6. Big Mom's history with Harald, there's now way they didn't have serious conversations over the years when Rocks, Big Mom's Captain, was Harald's best friend for years
7. Big Mom and Luffy still have yet to settle their score
8. Brook hasn't seen Big Mom's panties yet, this one is very important /s
So... yeah, a good amount.
Since Big Mom is an agent of chaos, I feel she's going to show up when things are most chaotic... so, around now. We shall see.