The only possibly interesting narrative for Belle and Sage that I can see that actually plays into Belle's development and isn't just "what do you have that I don't" is to have Sage be trying to take control of Belle and the other Badniks that she's managed to salvage (hell even bring in Omega and Gemerl while you're at it) to return them to fold while Belle actively and willfully opposes any attempts to force herself and other former Eggman robots to rejoin the empire.