People playing PokemonGo kept draining their phone battery.
Many people were searching for their first portable charger, but there weren't any batteries that were interesting, or designed specific to the game. People also wished they had batteries on them while they were outside.
Make batteries that look like Pokeballs and sell them outside.
I created prototypes for the first battery to be shaped like a Pokeball only a week after the game had launched. It had a built in keychain so you can attach the battery to your belt loop or backpack, and recharge your phone 2-3 times. I made these to sell in the street, and sold out within moments before putting them online.