So this may be a little late but I spent some time adjusting the Knight Fighter subclass a bit to be a little more unique.

I've really only adjusted up to 7th level but I am pretty happy with it personally and was curious as to what you all think.

Let me know and feel free to adjust or comment!

*Note* I did use some of the language from the recently released Knight Unearthed Arcana.

Knight

The Knight is a colossus on the battlefield who can shrug off attacks and protect allies from harm. They stand vigilant, prepared for any scenario and are unmoving in the face of danger. Able to identify openings in an enemy’s defense and pushing through even when knocking at death’s door, these armored bulwarks strive to keep the rest of the party safe.

By the Sword

As a knight you know that weapons are not merely tools. They are the brothers in arms that stand by your side in the midst of battle. Should they fall you may soon follow and each one has a story to tell. When you choose this archetype you are able to discern how a battle may have played out by examining the weapons that were used. You gain proficiency in History. Additionally, you may choose a favored weapon. If you score a 1 with this weapon, the weapon does not break upon critical failure. You must complete a long rest before regaining use of this ability. This does not apply for Magic Weapons.

Implacable Mark

At 3 level, you excel at foiling attacks and protecting your allies by menacing your foes. As a bonus action choose a target as your mark. A creature ignores this effect if the creature can’t be frightened. The marked target has disadvantage on any attack roll against a creature other than you or someone else who marked it. You may choose up to 2 creatures to mark. This increases to 3 creatures at 10 level.

If a target marked by you is within 5 feet of you on its turn or makes an attack that suffers disadvantage from this feature, you can make one melee weapon attack against it using your reaction. This attack roll has advantage, and if it hits, the attack’s weapon deals extra damage to the target equal to your fighter level. You can make this attack three times, and you regain all expended uses of it when you finish a short or long rest.

At The Ready

At 3 level, your trained vigilance and unwavering focus allows your companions to rest soundly while you stand watch. When setting camp, you patrol the perimeter searching for potential weak areas of security. If the rest is uninterrupted your allies can expend one free Hit Die. If interrupted your enemies do not gain a surprise attack and your allies receive advantage on initiative for the encounter. You must finish a short or long rest before regaining use of this ability.

Unwavering Guardian

Your instincts have been sharpened to protect your allies within a moment’s notice. At 7 level when a creature makes a melee attack against an ally you can see within your normal sight range, you may use your reaction to dash to that ally to stop the attack. If the creature is medium size or smaller the creature must make a Strength Saving throw. On failure the creature is marked by you and no damage is dealt. If the creature is successful, you take half of the damage dealt as if the creature had hit.

@Hazel, your adjustments look very interesting. Admittedly I haven't played the Knight Archetype, but I just had a player select it. (Yes I'm trying to incorporate UA into my campaign where it makes sense). My new player wanted to play a tank-type character.  I think I'll incorporate your changes. Unfortunately our next playing session won't be anytime soon or I'd be able to give some feedback sooner rather than later.

Thanks for sharing.

Hazel, I quite enjoy abilities you have here. The Unwavering Guardian, Implacable Mark, and By the Sword look amazing. The only thing I would be concerned about would be the lack of any amor/defense ability for the knight itself. All the abilities you outlined make the Knight the preferred target for the enemy, and yet the knight is not anymore tanky than say a monk. Furthermore, the ability At the Ready is situational. It is a great ability if you have to face ambushes to your camp, but I do not really have to face that problem often. It is a great ability but I think if you wanted, you could replace this one with some kind of defense buff of some kind. Maybe have the knight take 5 less damage when the total damage incurred is over ten. For example when taking 10 damage the knight would take 5, when taking 11 the knight would take 6 etc. There are plenty of other options for defense buffs as well. You could even make some kind of 11th level ability where the knight has some kind of area he/she can defend and any ally in that area gains some buff. As for the At The Ready ability, I think maybe you can make it so it actually shortens the long rest and short rest period. You can say that because the allies know the knight is guarding them they get extra good sleep and the short/long rest period gets shortened. This is in addition to what the ability does according to your description. 

This makes a few assumptions about mechanics that don't exist. By the Sword being the biggest offender, but you should re-check your rules so that they fit with the base game and make sure you price the benefits accordingly. 

For the By the Sword, perhaps, the benefit should be a roll of a 1 does not mean it is an automatic failure when a Knight is using his favored weapon. If a DM does employ some sort of critical failure other than the automatic miss it would be up to that DM to extend the benefit to their house rule of course. In my own game a natural 1 is such a horrible swing that it pulls the person out of position in such a way as to provoke an opportunity attack. I would consider extending the rule of a natural 1 not be considered a critical failure for a Knight using his favored weapon (i.e. a Knight would not provoke an opportunity attack when a natural 1 is rolled). However I would also make the favored weapon be a specific weapon and not a weapon type. The Knight would have to go through a ritual similar to attunement for magic items to make a weapon their favored weapon and they could only have one favored weapon at a time. Although I would consider adding an additional favored weapon at higher levels but I am not sure about that.

That would probably be better. Even I sometimes forget that basic-seeming things like Feats and Magic Items are technically variant or optional. 

I think the purpose of the UA Knight is actually perfect for the lore behind the build. Though the idea of mounted warrior is kinda lost without giving automatic proficiency in Animal Handling at level 3 (you can take it at level 7, but the first level 3 ability is called Born to the Saddle and you don't get free Animal Handling?). 

As for your changes:

1. As someone already pointed out By the Sword does things not currently in the 5e system (Critical Failures are only house rules). I do like the idea of the favored weapon though (after all historically warriors named their weapons). Since you have three level 3 abilities, you might want to move that to level 7 and instead have it be a combination of the Eldritch Knight's bond and the Pact Blade Warlock: "At 7th level, you can perform a ritual to create a magical bond between yourself and any non-sentient, non-artifact weapon of your choice. The ritual takes 1 hour and can be done during a short or long rest. The weapon must be within your reach throughout the ritual, at the conclusion of which you touch the weapon and forge the bond. Once the ritual is finished and the bond is forged, the weapon counts as magical for the purpose of overcoming resistance and immunity to non-magical attacks and damage. You can no longer be disarmed of that weapon unless you are incapacitated. If it is on the same plane of existence you can summon that weapon as a bonus action on your turn, causing it to teleport instantly to your hand. You can only bond with one weapon at a time with this ability. The bond breaks if you die, perform the 1 hour ritual on another weapon, or perform a 1 hour ritual to break your bond to it."  
2. I think Implacable Mark works better thematically by using attacks to mark rather than declaring a mark as a bonus action. You can still mark two targets, but you have to attack them to do it.
3. At the Ready seems like it's very situational, but it provides a very high reward for that situation happening. One free hit die after every short rest or not being able to be surprised AND advantage on initiative checks for the whole party? I would probably change it to something along the lines of "Anyone who is awake during a short or long rest has advantage on Wisdom (Perception) checks and can not surprised." This would mimic knowing where the weak points are in the camp's defenses and being able to spot someone in them quickly enough. 
4. Unwavering Guardian seems like it's fairly overpowered, what with the sprinting all over the map as a reaction and COMPLETELY negating the damage dealt? However, you could simply change it to be like the Phalanx ability that I have for my Lancer's Legionnaire archetype.
   
   Below is the archetype for my DM's Guild Lancer class which could work for a fighter - obviously the naming is more based on the Roman Legionnaire, but a Medieval Knight would also work for most of these:

Legionnaire
The Legionnaire is a lance wielding warrior known for leading armies into battle. Often the first line of defense against an oncoming horde, the Legionnaire is a warrior who uses tactics and sheer willpower to prevent the onslaught of forces from attacking those he or she defends.

 Phalanx
Beginning at 3 Level, your ability to defend your allies improves. As long as you are wearing a shield, any ally within 5 feet of you can add your shield’s AC bonus against attacks from enemies within your reach.

Also, anytime an ally within 5 feet of you is targeted by a spell or attack that requires a ranged attack roll that doesn’t include you, you may use your reaction to become the target of that attack instead. The attacking creature gains advantage for that attack. You must use this ability before the attack roll is made.

 Hold the Line (terminology is the same as the UA Knight, but effect is completely different)
Beginning at 7 Level, you can add half your proficiency bonus (rounded up) to any Strength, Dexterity, or Constitution check you make that doesn’t already use your proficiency bonus.

In addition, when you are subjected to an effect that would move you, knock you prone, or both, you can use your reaction to be neither moved nor knocked prone.

Strategos
At 11 level, your ability to lead from the front lines emboldens your allies. Whenever you use your action to use either the Dodge or Help action, you can use your bonus action allow an ally to attack. When you do so, choose a friendly creature that can see or hear you and that creature can use its reaction to make one weapon attack. That creature has advantage for the attack.

 Final Bastion
Starting at 15 level, when you are reduced to 0 hit points and are not killed outright, you can choose to drop to 1 hit point instead. Once you use this ability, you can’t use it again until you finish a long rest.