It’s hard to achieve a good screen/case ratio in DIY projects. The screen components are bulky and it’s difficult to fit everything in small enclosures without producing custom boards, mass produced hardware has a ton of advantages there.
It would be cool if someone made a mass produced "watch" that's all the hardware but none of the firmware. So a tiny computer on your wrist that doesn't do anything out of the box, and you can make it do whatever you want. Maybe a watch, maybe something else. It just provides the housing, microcontroller, battery, screen, etc., and an SDK.
Sensor Watch. Open source replacement movement PCB for the Casio F‑91W and other similar watches. Open source firmware. ARM Cortex M0+ microcontroller that you can develop for using standard GNU toolchains. Battery lasts years. Temperature sensor makes it a temperature compensated quartz watch. They're even making custom LCDs now.
Wear mine every day. Contributed some improvements to the pulsometer and TOTP faces. Was even maintainer for a while. Really nice community. If you want something awesome to hack on, this is it.