I demand BangleJS3!

Thanks, yes that's about where I am with it as well - Bangle.js 2 does have a membrane, and it does seem to work ok - but it's still something you can't really rely on for proper submersion (like swimming).

Heart rate is a hard one for me to add on a custom-designed watch, but I do think it is expected of a wearable at the moment so probably is needed.

maybe some option to reverse the watch interface(like text&images) if someone would like buttons to face another direction

Bangle.js 2 does do this already (well, it allows the screen to be rotated) - check out the LCD page in Settings