Reminder: Feedback is an ability that triggers when a spell or ability targets the card
While not specified inthe original task, I have interpreted the mechanic as activating before the effect of the actual spell or ability happens.
Ideas:
Creature, Feedback: Duplicate the effect on a random target on the same side of the field (Upped one would be on the on the opposite side of the field)
Creature, Feedback: Destroy this creature (something with high attack)
Weapon, 1 quanta ability: Target anything and generate 1 quanta of the element of that card. Feedback: Gain +2 atk (The ability can be used as an easy Feedback-trigger on other Feedback card and itself)
Creature, Feedback: deal 1 dmg to all other creatures
Creature, Feedback, gain +0|+4 (cc protection)