Can any Apple watch bands add extra features?
I have a vague recollection of several attempts at "feature" bands when the original Apple Watch was released but I'm not sure many of them even made it into production. The only one I'm familiar with that is a shipping product is the KardiaBand.

https://store.alivecor.com/products/kardiaband

It adds ECG features to Series 3 watches and predates similar features in the Series 4.