Looking through my list the only two I (barely) use are PCalc and DarkSky, which if I didn’t have, I’d just use the stock calculator and weather apps. I’ve tried maybe 20 other apps and none are compelling.
A jailbreak on the Garmin devices that enabled Fenix Pro features on the entry-level models would also eat the market.
I actually played with the TI 430-Chronos smartwatch development platform [2] a few years ago, but all the interesting features to me are tied to GPS.
Note: I currently am coveting the Fenix 6 Pro for my hobbies and covering it for hitting the right point in the Casio-F91 to smartphone-on-my-wrist continuum. I just wish some of its software features were available cheaper.
[1]: https://www.joelonsoftware.com/2004/12/15/camels-and-rubber-...
https://aliexpress.com/item/4000527495064.html
It looks like ESP32 + display + real-time-clock + gyroscope/accelerometer/compass + battery + buttons, all inside fitness tracker body. Just waiting for whatever you can code it into