So initially, my proposal for improving the Champion was to add +2 to damage in addition to their expanded crit range. For me, this made sure that the Champion consistently felt the usefulness of their ability, since theoretically even with an expanded crit range you could end up never landing a crit. I also felt that Remarkable Athlete could stack with proficiency. But now, I have a different idea.
I'm considering at level 3 Improved Combat also allows the Champion fighter to use their bonus action for grapple, disarm, and shove attempts. This is similar to the Rogue's Cunning Action, giving the Champion fighter something to do with their bonus action if they don't use Two-Weapon Fighting.
Level 7 Remarkable Athlete stacks with proficiency.
Then At level 10, in addition to a second fighting style, a Champion adds half proficiency on any saving throws they aren't already proficient in.
Finally, at level 15, Superior Combat includes the following add-on: In addition, whenever you attempt an ability check using Strength, Dexterity, or Constitution, you can treat a d20 roll of 9 or lower as a 10.
I feel like these additions are not necessarily overpowering. It makes the Champion more strategic in greater options with their bonus actions (also synergizes with expanded crit), tougher (in their saves), more athletic (potential for 1.5 proficiency in Athletics/Acrobatics), and more successful in their athletics (by taking 10 with physical skills).
What are people's thoughts?