I've been playing Aatrox recently, and I'm absolutely LOVING his kit. It rewards good positioning and accuracy extremely well, while also being straightforward to learn. The phrase "Easy to learn, Hard to master" fits Aatrox to a T.
However, it seems that he's not having much success outside of professional play. According to League of Graphs, his win rate puts him in the bottom 10% of champions at Platinum ELO, and he doesn't seem to fare too well even in Master rank play, with a win rate of just 45.5%. Based on this data, I believe it would be appropriate to reduce the Skill Floor required for him to be effective, so that more players can actually use him effectively.
So, before we talk changes, let's talk about what makes Aatrox, well, Aatrox. He is a high skillcap, physical-damage-oriented juggernaut who specializes in dealing massive sustained AoE damage to the enemy team under the correct circumstances, along with healing himself the more damage he deals. He excels in the early and mid game, but falls off late. His weaknesses include being kited, or being bursted down faster than he can heal (his tankiness without lifesteal is not very good, particularly in the physical department).
However, the hidden weakness that surprised me the most was other sustained-damage oriented melee fighters. The reason why Aatrox is weak to these champions in particular is that his melee-range damage requires setup. You have to cast the Darkin Blade twice before you can unleash a full-damage melee-range attack with it. The first two attacks actually deal 50% increased damage at different ranges outside of melee. If you don't believe me on this point, just look at the surge of players taking him Mid instead of Top, where he's dominating ranged matchups. Also, his matchups against ranged champions like Gangplank and Vladimir are extremely good, with win rates of +4.9% and +4.5% respectively at Platinum ELO (if you're wondering about Vayne, Aatrox's win rate against her is -1.8%, but it's not nearly as bad as say the Fiora matchup, where Aatrox sits at -3.9%). I don't necessarily think these champions should hard-counter Aatrox, given that he already gets kited pretty bad by ranged champions.
So, with all of the data presented, I think that the question we should be asking is "How do we make Aatrox more effective in Platinum+ play, without removing his counters or buffing his professional presence too much?"
The fact that Aatrox already deals so much damage when played right means an AD buff is out of the question. Buffing his AD would cause his professional winrate to skyrocket, which is what we don't want. The problem I think we need to solve instead is how to make his damage just a little more reliable in Platinum+ play. I believe that one of the ways to solve this problem would be by increasing his survivability toward the mid and late game. I know I said that late game is supposed to be his weakness, but I believe that he'll still be vulnerable to AD carries even if we buff his survivability. If he gets focused down and kited properly, he WILL still die. Now, Aatrox's natural lifesteal makes increasing his health pool too much a bad idea, given that it will allow him to heal for far more than he would normally. Think of the extra health like a Bloodthirster's shield: it makes him far less prone to being bursted down, while also improving his ability to sustain himself in fights, due to his natural lifesteal. Buffing his magic resist is also a no-no, given that he has one of the highest base magic resist values in the game. His early game is already obscenely strong in the right hands, so I wouldn't buff his base armor, either. With all of this in mind, I believe that buffing Aatrox's armor scaling is the way to go. As it is, he's frailer than Pantheon in both the early and late game when it comes to armor, so I believe there's some room to increase his scaling a bit without making him OP.
Personally, I would start with a small buff to his armor to make him a bit more forgiving when it comes to mistakes in the mid to late game, like this:
Current Armor: 38 + 3.25 per level (93.3 at level 18)
Modified Armor: 38 + 3.50 per level (97.8 at level 18)
However, I don't think that this alone will be enough to make him viable. I believe that we need to specifically address his melee-range problems in order to truly make him worth playing.
So, how would I fix Aatrox's problem against melee-range fighters beyond buffing his armor scaling, without making him OP in the hands of a skilled player? I believe that the answer lies in Aatrox's Attack Speed. Because Aatrox cannot hit the edge of the first two casts of the Darkin Blade against an enemy in melee range, he'll be relying more on his basic attacks for sustained damage and lifesteal. Buffing Aatrox's Attack Speed scaling will increase his effectiveness in melee range, allowing him to weave autoattacks into Darkin Blade's internal cooldown more effectively at higher levels, without increasing the damage of the Darkin Blade itself, which would be obscenely effective on champions at range. I would do it like this:
Current Attack Speed: 0.651 + 2.5% per level (0.928 at level 18)
Modified Attack Speed: 0.651 + 2.75% per level (0.973 at level 18)
Finally, I'd like to address Deathbringer Stance. While the minion healing does make him more forgiving, I believe it needs to go in order to buff Aatrox any further. To replace it, I would add a secondary Blood Well bar to Aatrox in place of his ult timer (the ult has other visual and audio indicators, so I think we could safely remove the bar for it). He would passively gain 4-8 "blood" per second, based on level. Hitting an enemy with an auto or the Darkin Blade would grant 8-16 "blood", and the edge would increase it to 16-32 "blood". This would make it much easier to keep track of Deathbringer Stance, which is one of the key points in determining when to trade with him, and I think this would actually serve as an quality-of-life buff below Master, and actually increase his winrate.
To summarize: Deathbringer Stance's cooldown is the same, it's just tracked by the secondary bar in a builder-spender fashion in place of his ult bar.
All in all, I think that these three changes would be a good place to start looking at making Aatrox more viable in Platinum+. However, keep in mind that I'm only Silver myself, so take my analysis with a grain of salt.
What do you think? If you wish, let me know what you think of my ideas in the comments below. Thank you for reading my analysis, and have a great day!
EDIT: Decreased the suggested armor buff from 3.75 per level to 3.50 per level. In retrospect, 9 base armor at level 18 might be a bit much. Instead, the Attack Speed buff should give Aatrox some extra sustain in melee-range skirmishes, while keeping his main weakness of being kited more intact (keep in mind, Deathbringer Stance's cooldown is reduced by basic attacks as well as Darkin Blade, so buffing his attack speed would be a much larger buff than it looks)
EDIT 2: Floating a new passive for the Darkin Blade (Q). See the section right under the Attack Speed buff for details
EDIT 3: Decreased the Darkin Blade passive attack speed increase from 50% to 33%. Even accounting for the 1 second internal cooldown on Darkin Blade, +50% looks way too high to me.
EDIT 4: Getting rid of the Q attack speed buff entirely. It just adds more complexity to his kit, and his E already has an autoattack reset. I simply no longer think it is necessary. Also, added a proposed QoL change to Deathbringer Stance that I believe will make him easier to play with no actual gameplay changes at all.
EDIT 5: Changed "Fury Bar" to "Blood Well". Thanks Gelsominokiller for the suggestion!