Chaosmancer
Legend
Many of these suggestions keep hitting on what the Fighter is already great at - dealing and taking damage. They are an A-S tier class in both departments. Weapon mastery offers a significant damage buff and a few more combat options, though nothing too exciting. They need more options outside of combat, though that has to come at a cost - no class can be A-S tier at everything.
I think what limits fighters is that they are really great at these two very important and pretty easily quantifiable things. So they have a very clear role. But they are bottom tier at everything that isn't those roles. This is a significant design problem, because it can play out very differently at different tabletops. If you have a combat-heavy, roleplay-lite table, fighters are amazing. If you have a roleplay-heavy, combat-lite table, fighters suck.
Contrast with, say, a rogue. Rogues are A-S tier at exploration, and probably C-B tier at everything else. Rogues are an easier class to balance; you can tweak a few things, but players at most tables will find opportunities where their rogue can be the star at times.
I argue that increasing damage output or survivability works against making fighters more interesting and flexible - they don't need buffs in those departments. Any improvements should be in areas that make them interesting outside of combat. This is potentially addressed to some degree, by the greater access to feats that fighters currently have, though a lot of players don't really look at feats as a way to broaden the capacities of their fighter outside of combat, but more as a way to double down on what the fighter already does well.
Explain the Paladin and to a lesser extent the Ranger then.
A 20th level Paladin, with the current rule set can rather trivially do the following damage. Utilizing Polearm for consistency with fighter numbers, and since it has always been insisted to account for subclasses, let's make this a devotion paladin. The DEFENSIVE paladin option. Channel Divinity used for +5 to hit
0.85x16x2 = 27.2
0.05x16x2 =1.6
0.85x16 + 0.05x16 =14.4(Cleave)
0.85x13 = 11.05
0.05x13 = 0.65
0.98x4.5 = 4.41 (Charger)
0.98x6 = 5.88 (GWM)
0.85x16 + 0.05x16 = 14.4 (Polearm reaction)
0.98x27 = 26.46 (Divine Smite)
0.6 x 9.5 = 5.7 (Steed's turn)
+11 Aura no save
122.75 damage for the round
And they are just as tough as a fighter (maybe tougher), boost their allies saves, gave an ally ~9.5 temp hp during all this, and have a lot more resources they didn't use. They have spells that allow them to do quite a lot socially, and can reshape the battlefield.
Champion fighter? Same feats and build? I'll even give them Action Surge.
0.6x11.5x8 = 55.2
0.15 x 11.5 x 8= 13.8
0.6x11.5 + 0.15x11.5 = 8.625 (Cleave)
0.6 x 8.5 = 5.1
0.15 x 8.5 = 1.275
0.6x11.5 + 0.15x11.5 = 8.625 (Polearm Reaction)
0.84 x 4.5= 3.78 (Charger)
0.84 x 6 = 5.04 (GWM)
101.45 Damage for the round.
And sure, you can say "but the Paladin used resources" but so did the fighter. I used Action Surge. And I couldn't even match what the paladin did with only a single smite and their capstone. Now, the champion is tougher, most definetly... but they aren't more versatile. They aren't buffing their allies, they can't heal their allies, they don't have the options of a spell list... and they don't do as much damage.
So if the Fighter is S tier damage... what is this paladin? This Paladin that is NOT the best damage dealing paladin. They are the DEFENSIVE paladin? Why can the paladin reach this level, and still have so much, but the fighter is below them and can't have more?