I'm mostly interested in specialty/niche sports/fitness applications. All of the built-in stuff is great for people who think running is where fitness ends or need to be reminded to walk every day, but not so great for someone looking to improve a 600lb deadlift or cut their 40m time.

I also think a handful of other fun apps could be profitable, like using the local communication thing to send shock images/reaction gifs/dumb sticker packs.

The first IAP Tamagotchi-type game will make a million dollars easily.

I'll probably stick to focusing on health and performance, but plenty is possible.