A fan is overkill for a Raspberry Pi. A passive cooling is sufficient, and most of all, absolutely quiet.
A very good case is the "GeeekPi Argon Neo Aluminium", which I recommend
I agree; If an aluminum case were designed to integrate with built-in heatsink, thermals should be improved enough with no downside to noise level. Their decision to opt for plastic in the first party case makes total sense from a manufacturing and cost point of view, but I think a more premium metal case would have also been a nice accessory in addition.