Ever since its introduction, the S Pen has been the defining feature of the Galaxy Note series. Some users are so devoted to that stylus they won't even consider buying another phone. The S Pen hasn't changed much over the years, other than no longer getting stuck in the phone. Samsung seems to have something big planned for the Note 9, though. A new FCC report lists the S Pen as a Bluetooth device.
The FCC is tasked with testing consumer electronics to ensure they don't interfere with wireless communication. The Note 9's S Pen (model EJ-PN960) got a full battery of wireless tests because it's a 2.4GHz Bluetooth LE device. Past S Pens have been unpowered inductive styluses that interact with a digitizer in the screen. Adding Bluetooth could unlock a whole range of new features (and a few drawbacks).
Samsung will need to work out a way to keep the S Pen charged. There's not much space for a battery, but it could probably trickle charge the pen whenever it's docked inside the phone via pins or wireless technology. With a Bluetooth connection, the button on your S Pen could work even when it's not near the screen. It could potentially control music playback or open Bixby (please no). We'll have to wait and see what Samsung has cooked up.