My Ideas to tweak Legion. Feel free to poke holes so we can come up with a possible Rework
So a lot of people on both sides of DBD are at odds with the Legion. Killers know he's brain dead to play as, and Survivors have no fun factor getting focused, especially with the Infamous Frank's Mix Tape. These are my ideas to make Legion fit into what his design intentions seem to be; a killer capable of map pressure and punishing survivors for grouping together, instead of his current form where if they decide to focus one person, there's nothing they can do.
Their M2 stabs are no longer lethal (but still take down the survivor one health state) in line with the "Thousand Cuts" theme. With every stab it applies 3 debuffs in sequence: -5% gen repair, -5% to You being healed, -5% to You healing Others. for instance on your 4th hit it would be -10% gen repair and so on. The mend action would be for removing these debuffs. Completing the Mend action will drop 2 debuffs, as this is the standard amount they can place on you with one activation of their ability. Now one thing people asked a lot for Legion's buff would be to simply to make them a 115% movement killer, but I personally feel any addition to killer movement speed requires work: If there are enough stacks going un-mended Legion gains 5% movement speed. This effect could be announced on the map similar to Myer's tiering up his Evil Within.
Since some of their addons are based on the bleedout bar, you need to change those as well. The aforementioned Franks Mix Tape now makes it so you can apply 3 debuffs before being stunned. The Stab Wounds Study could instead increase the intensity from 5% per stack to 7% for the debuffs.
I think with these changes Legion can't cheese you with chase mechanics to down you with Deep Wound. Also since his ability wouldn't have this effect, Borrowed Time can go back to applying the Endurance buff and its old effect.
So overall I'm not sure how these changes would play out with his overall Viability as a killer, but would make him much more enjoyable to play against. Yes Mend time is still just holding M1 which many survivors feel isn't enjoyable, but if stacks of the debuff applied were a part of the killer's strength, they can think to themselves, "yeah Legion let's see if you can get that 5%".
Feel free to challenge or come up with possible What If's if you see something I didn't with this idea more point of views will make it better.