Just chiming in to say that I'm also ready for a Bangle.JS3. My JS2 is getting scratched up and the battery doesn't hold a charge as well as it used to, and I'd prefer to upgrade rather than just get a replacement.
As for what I'd want changed, I'll just have to repeat what other people have said and say that I'd like button controls, more colours and better waterproofing. What I don't need is a GPS and barometer. My phone has a GPS already and I've never felt the need to have a second one on my watch, and I don't know what I'd use a wrist-mounted barometer for in the first place. The heart-rate sensor is nice, though.
This could probably be added to the JS2 through a patch, but I'd really like to be able to configure my watch apps remotely. Dealing with the Bangle's tiny touch-screen is a pain, and it would be really nice if I could do most of my interaction via Gadgetbridge or another app so that I have a larger interface to work with, and mostly use the watch for output.