Imagine a remote village with a remnant of an ancient tradition. Every decade of life, a person gets a cure disease. End to parasites, cancer,etc. As long as one does not meet an accident, or disease they can’t survive, hey will reach their maximum age.

As a remnant of a past age there is only a 5th level priest who can only do one 3rd level spell per day, baring an 18 wisdom and time to rest and relearn spells.

Twins and triplets would be a challenge as oldest gets the day, etc. Get 23 plus people together and 2 of them will share a birthday. (I had a logic professor who proved this in every class.)

I see an overworked aging cleric with a young assistant not up to the task.

The aged would be honored. Death would have a grand ritual to show the deceased into the nextv realm.

In ancient times, all healing spells would be available. Healing, restoration, disease, etc.

A lost or hidden town or city where this is still the case would be a fun adventure.

Customs and norms a party would wreck, or be a welcome relief.

A healing fountain that is broken or lost to living memory would fit well here.

My inspiration:

I got home from surgery for prostate cancer and would love a cure serious wounds to get some energy back. Then I realized cure disease would have handled the whole thing. My mind naturally wandered to an adventure scenario.