Backstory: Whenever we visit the grocery store, Julia flips through a stack of 5 credit cards and picks a seemingly random card to pay. I asked her what she's doing, she mentioned that card has a special this month, double your points on grocery store purchases. She loves couponing and getting sales, it's a hobby of sorts.
The Idea: An app that would tell you which credit card to use when visiting a store.
Deep Dive: The average American that has at least one credit card, has at least 3.7 cards. You would take all of your cards and enter them into an app, the app would determine which credit is the best to use each month at commonly visited places, grocery stores, gas stations, flights, etc. You could get fancy and geofence this but in the beginning just send a monthly reminder and give the user a simple login to view all of their credit card savings. The app could also alert you if any card was doing a point bonus near the end of the month.
Validation: This idea would piggyback off couponing apps like honey and savings. It would certainly save Julia some time at the checkout.
Monetization: You're tapping into user spending habits. Keeping this app free as long as possible is key because the data is worth a lot. You could sell ad space to credit cards with bonus points/signups like nerdwallet.