Asklemmy

While I agree that instruments should generally aim for sound and ergonomics over appearance, those things don't always have to clash with each other. To take Selmer clarinets as an example, since you said you liked them, it's normal for the keys and posts to have a highly-polished silver appearance, but you can get the signature model with matte black keys and posts if you want. That's a totally visual choice that won't change anything else, but it makes for a really distinctive-looking clarinet. Same goes for all the decorative inlays in the wood.

Besides that, though, I don't necessarily mean the visual appearance of the instrument. Check out the synth that the other reply to me linked. It barely has any concessions to aesthetics at all, but someone has clearly experimented with ways to get a highly-personalisable playing experience. My dad and I are both guitarists, but we set our instruments up for totally different sounds because people experimented with stuff and we got to follow in their footsteps to pick what we subjectively liked best - the clear treble sound of thin roundwound strings for him, the rich warmth of thick flatwounds for me. Those are also things that came about because someone, at some point, had the curiosity and ability to just try it out and see what it sounded like, and I love that we get to enjoy the fruits of so many of these experiments now.