Retail app development opportunity: In-store geolocation
The devices don't need to be limited to ads. One simple application can help guide humans to what they want to buy. Already some warehouses have LEDs that flash to guide the humans packing orders. A store can have a similar system that interacts with any app to help people find products without searching and searching.
It's important to recognize that the retail API does not need to interact with the human. Smartphones are constantly broadcasting their ID numbers in the clear and some stores are tracking customers to help plan store layouts. A savvy API might simply detect and identify the human from the smartphone signals and then reconfigure the store experience.