Backstory: When traveling, it's always nice to use a prepaid credit card incase you lose your card. There are a ton of other situations where a prepaid card makes sense. You don't know how much money is left on the card unless you call a number on the card or keep a record.
The Idea: Color coded led on a prepaid credit card indicating how much money is left.
Deep Dive: This one requires a physical product that has a long battery life. Similar to tile, you would need to have a small battery source to keep a small led lit, indicating how much money is left on the prepaid credit card.
As you progress with this idea, you would build a mobile app so you can configure the colors to be custom.
Validation: Currently there is no easy way to tell how much money is left on your prepaid card.
Monetization: You could have default colors and charge users a monthly fee to edit the colors. As with any prepaid situation, there is money to be made when you're charging an extra amount to use your services.