What if Chidori and Raikiri had been two separate ninjutsu?

The A-rank Chidori would be pure shape transformation which concentrates a large amount of chakra into one’s hand until it physically manifests as a destructive mass that sounds like the chirping of a thousand birds. By charging at the target at top speed – ideally dragging the ninjutsu against a wall or the ground to harness extra energy from the friction – it becomes the blade of a spear which is thrust into the target to pierce through their body in a one-hit kill.

Kakashi invents the Chidori when he was merely 13 years old in an attempt to match his sensei’s Rasengan. But because the ninjutsu is significantly weaker without the right level of momentum, it needs enough speed behind it that he experiences tunnel vision to kill which makes it too difficult to see incoming attacks or respond to an enemy’s evasion. By getting the Sharingan, he compensates for that weakness but it doesn’t change how the ninjutsu fundamentally worked.

Years later, Kakashi combines Chidori’s shape transformation with nature transformation. By concentrating lightning chakra into his hand, it creates an inherently more powerful ninjutsu which produces even greater destructive force, ravages the target’s insides with electricity and can release streams of lightning. Most importantly, it can be used without needing to run at all and still delivers a lethal blow even from point blank range.

The first time he uses the improved version of the Chidori in battle is during a massive thunderstorm. The ninjutsu accidentally acts as a lightning rod as Kakashi charges at an enemy but the lightning bolt is severed and he succeeds in killing his target. The enemy forces ran at the sight of such a feat and word spreads of Hatake Kakashi’s “Lightning Cutter”.

So Kakashi decides to simply call his new S-rank ninjutsu, Raikiri.

He would go on to teach Sasuke the original Chidori during the Chuunin Exams as his Sharingan would allow him to overcome its weakness. After Sasuke leaves Konoha, he masters lightning natured transformation and learns the Raikiri for himself. He stills develops a number of original lightning ninjutsu but they aren’t explicitly based on the Chidori itself.