To be honest, what would a mid do for Killer anyway? He's got the godlike Berserker Stance 1, which is a great mid. If he was given another mid I think it'll be overkill. If Leatherface's pokes were buffed (hit advantage, not speed), I think he'll be fine.
If Berserker Stance 1 was +2... Well for a move that can do 38-40% raw that would be silly. Leatherface isn't a pressure character by any stretch of the imagination.
Killer isn't bad at all... His main weakness is not having a projectile, but he can absorb them with berserker stance cancels. If that window was increased a little bit that would be nice, but not vital. Most of the changes to Leatherface are universal.
Not trying to have a go at you at all but if he was given all these buffs he would be top 10 for sure. I just want a character that has a chance against everyone else in the cast. As
@Kokoko put it, maybe when (if) NRS decides to nerf the current top tiers I think Leatherface will move up the tiers. All he needs is a few quality of life changes and he's good to go. I was overstepping myself with my initial suggestions and after the Leatherface community have discussed these changes I realised how silly they were.