So after reading this thread and several others regarding fighters, I have decided to implement a few changes to the Fighter class in my games. Many of these have already been implemented (though no one has yet wanted to go fighter in my games, so play-testing has been nil), but a few are new ideas. I'll detail my changes, as well as my reasoning behind the change:
Level 2 - Shrewd Fighting
Level 6 - Action Surge
Level 14 - Action Surge (twice per day), Steel Resolve (Add half proficiency bonus to any save you are not already proficient with)
Level 17 - Extra Attack (4 attacks total), Indomitable (3 Uses)
Level 20 - Fighting Style Mastery
Shrewd Fighting - Starting at 2nd level, your skill and reflexes in combat have improved. You can take a bonus action on each of your turns in combat. This action can be used only to take the Grapple, Shove, or Disarm action.
Reasoning - I think the general chassis of the Fighter can be improved by allowing more options to utilize and encourage attempting things other than attack. I've seen many argue that creativity in role play can prevent this, but by outlining these action options within the fighters skills, rather than just in the Combat Actions section of the PHB, it brings it more to the front of the player's mind as an option. Also, Action Surge is an AMAZING ability and with that being at second level, the fighter is a popular dip class. Offering this at 2nd and moving Action Surge a bit later can discourage those dips.
Action Surge Changes
Reasoning - As I mentioned above, I think Action Surge makes Fighter dips very appealing. Not that dips are necessarily a bad thing, but I think such a powerful ability should be earned. However, while it is a great ability, I think waiting until level 17 to get it again is perhaps too long. Also, I don't agree that the Fighter should be the master of feats or ASIs. While that was the approach in 3e, I think it made more sense given the prevalence and power of feats, as well as how specialized feat trees could get. 5e has a much different approach, and so I am trying to have the Fighter distance itself from the 3/3.5 version.
Steel Resolve - Starting at 14th level, you can add half your proficiency modifier to any saving throw that you are not already proficient in.
Reasoning- Another ability that I saw as being worthy of taking the place of an ASI/feat. It is somewhat similar in power level to the Resilient feat, but rather than grant proficiency in one good save you get half-proficiency in 2 good saves (when I say good saves, I mean Con, Dex, Wis, in my opinion the others are of negligible value).
Extra Attack - The 4th fighter attack is great, but not worthy of a capstone ability. Especially if you compare it to a Cleric getting a personal favor from their god, guaranteed, or how a Paladin becomes the avatar of their oath. It's boring for a capstone. Also, it breaks the expected math. Cantrips get their 4th damage dice at level 17, and looking at the extra attack levels in the fighter table, they clearly set that up as the pattern but then for no reason artificially delayed it. So I have greated my own unique capstones based on the fighting style chosen by the fighter that expands on that style. I won't share the specific capstones, as they are still somewhat in development. But if you are curious I have posted them previously on the board before.
Indomitable - Now Indomitable allows the fighter to treat their save as if they rolled a 20. Nothing sucks worse than using Indomitable on a Wisdom save and still failing.
Battle Master Changes
Know Your Enemy - So with this one, it almost seems like the level 7 archetype ability is a ribbon ability. However, unlike the Champion and EK, this ability doesn't really have any impact on the game. It fits with the BM, for sure in being able to size up an opponent, but after 1 min or 3 rounds of combat you probably don't need that info anymore. The other abilities will get tons of play, this one not so much. So instead, my thought is to change this to Battlefield Mobility, allowing the BM to add their proficiency bonus to AC against attacks of opportunity. In my head, the Battle Master is the trickier, finesse-ier fighter that is going to work to move around the field. This way, its power is inline with the others, but it has a more observable and consistent impact in the game.
Relentless - I personally think you should get 2 dice instead of just one.
Champion
Improve/Superior Critical - I think this should become Improved/Superior Combat and in addition to the increased threat range, the Champion gains a +2 to damage each time they get this ability. This way, the Champion feels like their ability is always on, they don't have to rely on the fickle dice gods for their power to come through, and it maintains the simplicity of the archetype.
Remarkable Athlete - This should allow adding the bonus on top of proficiency if they have it. The fighter already gets so few skills as it is, it's not going to break anything and will make the ability feel more useful.
Eldritch Knight
Eldritch Charge - I feel that it is too limiting to only allow this ability during an action surge. With this limitation, it is much weaker than the same level abilities of the other archetypes. Personally, I wouldn't have a problem and think it would be interesting to just allow the EK to Misty Step at will. If you compare to, for instance, Warlock invocations, a misty step invocation would likely be placed at level 15, so it seems the right power level.