Well, the Galaxy S8 isn’t even out yet and already the first few software modifications for the device have started dribbling in. It seems that enterprising redditors have found a way to bring the Bixby button new functionality using the All in one Gestures app, allowing users to remap the key to other applications or inputs, such as Google Now.
My personal spiritual yogis at XDA have a fantastic write-up that details how to set up the app. It uses Android's accessibility service (and root permissions, if you have them) to intercept the KeyEvent triggered when the button is pressed with little to no perceptible lag, and redirect that action to one of the user's choosing, either launching an application or even other hardware KeyEvents like volume, camera, or power on/off, giving users a lot of flexibility in how to use our newly-freed Bixby button. The privacy-conscious should note that granting accessibility service access to an app is tantamount to root-level access in many ways, and should be considered with caution if you distrust the source of a piece of software (All in one Gestures has decent reviews, but it isn't open source).
Even if Samsung hasn’t provided us yet with a nice sanctioned means of changing or augmenting that functionality for our own uses, it just goes to show you can’t stop development.