10 Buffy The Vampire Slayer And Angel Plots That Were Never Resolved
4. Out Of Mind, Out Of Sight
So, the US Government has a program where they recruit invisible people and train them to be assassins. It is a fantastic idea - one that is never once revisited in the seven seasons of Buffy. That is not counting the tiny homage that the seventh season pays to Marcie, where Buffy slaps a girl back to visibility!
The power of the Hellmouth is such that once Marcie Ross started to feel like she was invisible, she truly started to become invisible. This led to her descent into insanity, consumed as she was with the desire to exact revenge on Cordelia - who she saw as the reason behind all of her misery. To be fair, she wasn't entirely off base there.
As to be expected, Buffy manages to overpower her, as Marcie's only 'power' is that she can't be seen. She has no super strength or fighting skills to speak of.
Enter the US Government.
They quickly take her off Buffy's hands, before herding her into a seemingly empty classroom. It is full of invisible people like her, all who are being trained on convert missions and assassination technique.
With the later introduction of The Initiative, this absolutely seems like an asset that should have been revisited in the show!