Thankvenge is the opposite of revenge. When somebody does something nice for you, and you plot and scheme a way to return the favor upon them, you have had your thankvenge.
A gift, huh? For me? Oh, I see how it is! A very NICE gift, isn't it? Well just you wait! I will bide my time, and when the moment is right, when you least expect it, I shall wreck my terrible thankvenge upon thee! You have been warned! Thankvengeance shall be mine! HAHAHAHAHA!