You have a Social Link/Confidant with a family and each rank let's you hang out with the family in their crazy antics. Would be similar to the old couple from P3 in how you interact with multiple people in one Confidant. The size of the family should be small though since that would be asking way too much.

Give the protagonist a personality and history, and have a Social Link be them visiting the grave of a deceased family member they were close to, and have them reflect on things... or just meet someone at a graveyard who is reflecting if you wanna be lame I guess. Honestly if Koromaru was allowed a Social Link, anything goes, right?

A social link with delicious pancakes maybe someone from a previous game? That would be pretty cool. Maybe you stumble upon Naoto because her detective work takes place near where the protagonist lives or something.

Hmm, my mind seems to get stuck on fishing! So perhaps a fisherman? But the twist is that he's really bad at fishing, and the only reason why he goes there is because it reminds him of a dead family member? Just spitting out ideas!

Are there going to be brainstorms about mementos quests too? Like, with the family confidant that llama suggested, we could have some greedy landlord wanting them to pay an insane amount of money or get kicked out of their house, which leaves them in a too poor state to ever move out of their house.

a car. I wanna hang out with miata-chan

Maybe Nanako (all grown up) is teaching piano lessons at the school During her Socal Links, she talks about Narukami and the events of P4