Champion (5A)

This is a Fighter subclass for 5A.

The archetypal Champion focuses on the development of raw physical power honed to deadly perfection. Those who model themselves on this archetype combine rigorous training with physical excellence to deal devastating blows.

Greyhawk: Champions are most often self-taught, commoners who advanced by skill and merit. Their background is likely to be militia rather than castle, city watch rather than mercenary company. Still, the very pluck and determination that drives a champion earns them respect once they prove themselves.

Source: Player's Handbook

Subclass Abilities

Improved Critical

Beginning when you choose this archetype at 3rd level, your weapon attacks score a critical hit on a roll of 19 or 20. When you score a critical hit, you gain an additional attack to use on the current turn.

Remarkable Athlete

Starting at 7th level, you can add your proficiency bonus half your proficiency bonus (rounded up) to any Strength Dexterity, or Constitution check you make. This bonus stacks with skill proficiency in Strength (Athletics). You can never add more that double proficiency bonus to an ability check.

In addition, when you make a running long jump, the distance you can cover increases by a number of feet equal to your Strength modifier.

Additional Fighting Style

At 10th level, you can choose a second option from the Fighting Style class feature. At 18th level you can select a third option. You cannot select the same option twice.

Impervious

At 10th level, opponents cannot make attacks of opportunity against you.

Champion's Might

At 15th level you can use Strength (Athletics) to grapple and shove opponents two size categories larger than you are.

Superior Critical

Starting at 15th level, your weapon attacks score a critical hit on a roll of 18-20.

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 if you have no more than half of your hit points left. You don't gain this benefit if you have 0 hit points.

Editor's Notes

As perhaps the most iconic, newbie-friendly, and easy to use subclass, the champion needs to be decently good, or new players might become alienated.

The main improvement is to Improved Critical—not only do you get a better critical range, you also inflict additional damage. This goes from a 50% damage boost to around a 100% damage boost.
Remarkable athlete stacks with proficiency in Athletics (improving grapple and shove). It no longer applies to Dexterity checks. This reduces the bonus to initiative.
At 10th level you ignore most opportunity attacks—which should allow the champion to get right to the main baddie, which I feel is very champion-like.
At 15th level you can shove and grab opponents up to Huge size.
Finally, Survivor's self-heal has been doubled.