In 2023 I was using WeavRun (a running app that syncs music to pace) and wanted real-time coaching when I slowed down.
Emailed them the idea, got a meeting with the Head of Product, and showed up with a working Swift prototype I'd built over the weekend. They offered to add me as a developer. Added a vibration mode for runners with limited hearing. Two years later Apple shipped something similar with Workout Buddy.