Here is a stab at Champ:
Improved Critical
[Unchanged]
Get Back Up
You can expend your reaction when reduced to 0 HP to use your second wind, if available. Your HP are set to the amount healed by second wind, instead of 0, and additional damage is ignored. When you use your second wind, you gain advantage on all weapon attacks, saving throws and ability checks until the end of your next turn.
Remarkable Athlete
Starting at 7th 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. If you are proficient in Athletics or Acrobatics, you may add your proficiency bonus twice to such rolls instead of once.
In addition, when you make a running long jump, the distance you can cover increases by a number of feet equal to your Strength(Athletics) modifier.
Practice makes Perfect
At 10th level, you can choose a second option from the Fighting Style class feature, and you can use Second Wind an additional time before taking a short or long rest.
Superior Critical
[Unchanged]
Survivor
At 18th level, you attain the pinnacle of resilience in battle. At the start of each of your turns, you regain hit points equal to 5 + your Constitution modifier if you have no more than half of your hit points left. You don't gain this benefit if you have 0 hit points. In addition, you roll all saving throws with advantage.
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I think this remains simple, but is now significantly stronger.
Second Wind has synergy with its crit-fishing. It gives a survival boost as well (reaction second wind) if you don't want to be aggressive. Using it "early" gives you more offence, saving it for ignore 0 is more defence.
The 2nd fighting style (aka +1 AC or similar) also gives another second wind use.
To Survivor I added advantage on all saving throws (including death saving throws). That with Indomidable makes this Champion harder to put down.
I was tempted to add a "when reduced to 0 HP by damage, make a Constitution saving throw against the damage taken; if you succeed, you are instead reduced to 1 HP" die-hard to it.
Improved Critical
[Unchanged]
Get Back Up
You can expend your reaction when reduced to 0 HP to use your second wind, if available. Your HP are set to the amount healed by second wind, instead of 0, and additional damage is ignored. When you use your second wind, you gain advantage on all weapon attacks, saving throws and ability checks until the end of your next turn.
Remarkable Athlete
Starting at 7th 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. If you are proficient in Athletics or Acrobatics, you may add your proficiency bonus twice to such rolls instead of once.
In addition, when you make a running long jump, the distance you can cover increases by a number of feet equal to your Strength(Athletics) modifier.
Practice makes Perfect
At 10th level, you can choose a second option from the Fighting Style class feature, and you can use Second Wind an additional time before taking a short or long rest.
Superior Critical
[Unchanged]
Survivor
At 18th level, you attain the pinnacle of resilience in battle. At the start of each of your turns, you regain hit points equal to 5 + your Constitution modifier if you have no more than half of your hit points left. You don't gain this benefit if you have 0 hit points. In addition, you roll all saving throws with advantage.
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I think this remains simple, but is now significantly stronger.
Second Wind has synergy with its crit-fishing. It gives a survival boost as well (reaction second wind) if you don't want to be aggressive. Using it "early" gives you more offence, saving it for ignore 0 is more defence.
The 2nd fighting style (aka +1 AC or similar) also gives another second wind use.
To Survivor I added advantage on all saving throws (including death saving throws). That with Indomidable makes this Champion harder to put down.
I was tempted to add a "when reduced to 0 HP by damage, make a Constitution saving throw against the damage taken; if you succeed, you are instead reduced to 1 HP" die-hard to it.