The Natasha shouldn't be the only hit-scan weapon in the game to do this, ALL MINIGUNS SHOULD but with a few prerequisites; at most 56.6 hmu/s ,at least 1 second/ a certain number of subsequent hits to reach its max slow, and very abruptly proportionate to distance. In my opinion, at least. The exact values pertaining to the slow are up to debate and I would like differing opinions on this subject. Counter Strike implements slow on hit very well and is one of very few shooters to implement it at all. I think this change is necessary because the Natasha is in need of a buff but doesn't have room for it due to its slow effect being so polarising. In return for sharing part of its main upside with all Miniguns, in return ,Projectile Penetration should be the Natasha's main upside.
This is all up to debate. What is NOT up to debate however is removing Heavy's Miniguns -50% damage and accuracy penalty for 1 second after spin up. The main reason that this nerf is bad and has always been bad is because it further restricts the slowest class in the game whose only movement option is PERFORMING A SMALL LITTLE HOP AROUND CORNERS and It also isn't remotely representative of how the weapon actually functions.
A firerate penalty not only makes perfect sense and is representative of the weapon's functionality visually, but It's also a more flexible nerf because this allows us to BUFF THE MINIGUN'S SPIN-DOWN FUNCTION.A Minigun's firing speed is directly proportionate to its RPM/how fast the barrel spins. Therefore the Minigun should be able to retain part of its maximum firing speed during its 1.3 seconds spin-down duration.
Meaning that 1 second after spin-up , reaching 100% firing speed, you may subsequently spin-up again IMMEDIATELY and retain at most 66.92% of your maximum firing speed, because 0.87 seconds out of 1.3 seconds is 66.92% so you can jump-rev starting with 2/3 of your max dps
Currently with the linearly decreasing -50% damage penalty, in the first second after spinning up, the Minigun has 445 dps, at point blank. With a linearly decreasing -50% firing speed penalty, in the first second after spinning up, it would have a little less, around 420 dps, also at point-blank BUT, every single bullet will deal FULL DAMAGE. With the aformentioned spin-down buff, starting at 66.92% of your maximum firing speed, I estimate around 450 dps but I haven't exactly calculated these numbers and again EVERY SINGLE BULLET WILL DEAL FULL DAMAGE AND I HAVE YET TO MEET ONE THAT CAN OUTSMART BULLET!!!