The side button in combination with the digital crown are also used to do a hard reset when it gets confused and doesn't operate as expected (press and hold both for 10 seconds until the screen turns off and it reboots). So it actually already does perform several different functions.
Single press - friends access
Double press - Apple Pay
Press hold with crown - hard reset
The only time I have used that button was to reset my watch. I hope for some customisation here in WatchOS 2.