Let me preface this first with the following: I have not actually played a straight champion fighter, I have played a champion fighter vengeance paladin, and I only every now and again regret not going battlemaster, but that's because my dice hate me and I rarely crit, even with advantage and expanded crit range. Sooooo, my suggestions may not be particularly helpful.
Made my buddy a champion, he wanted to be a dual-wielding halfling fighter, but wanted a quick character (he's played before in previous editions a lot, and 5th a little, but I spend a lot of time making characters so I can pump them out with some pretty serious speed), so we agreed he'd be a champion. Normally he plays rogues or more great-weapon oriented fighters. He was REALLY bored with the character. Basically his issue seemed to be that his options were to roll a bunch of attacks, and then roll some damage dice. As such, I want to offer a few potential options, suitable for 3rd level when you pick up the archetype:
-- NOTE -- I would only choose one of these options, or, if you really like more than one, tone them both down. Throw anything probably isn't going to break any games, so maybe it could be left as is and another one added at a higher level or something.
1. Throw Anything: the champion can throw any weapon as though it had the throwing property with a 30' range. Doesn't return, you can just throw it, using whatever ability is appropriate to the weapon. Now you have a reason to carry all those magical longswords that pop up in random loot that nobody will use...
2. Give them a choice of +2 Str, or +2 Dex at level three. This would free up the 4th level ASI to let them pick up a more flavorful feat, but still let them retain improved effectiveness. The fighter gets additional ASIs anyway, so it's not out of line for the class, but this locks them into a specific choice that influences their accuracy and damage directly increasing their role effectiveness. This would let them further specialize with their favored weapon(s), or pick up some action versatility, or just pick up some nifty ability.
3. The short rest abilities of the fighter now restore after leaving initiative, or, refresh when they roll initiative, similar to the "if the battle master rolls initiative without any superiority dice they get one back" feature, but at much lower level. This is probably the most OP one, but it would bring the champion closer to the always on 3rd ed playstyle for the fighter without having to fiddle with too much. These additional action surge uses would have the added benefit of letting them stay competitive with the battlemaster since it would shift, for them, the action surge mechanic from a rest recovery to an initiative recovery. I feel this also ties in to remarkable athlete very well thematically; "The party escapes the gith forces narrowly by leaping onto a shipping barge heading out into the astral sea. As they prepare to rest and formulate a new course of action, the cargo begins to move and a slimy tentacle slithers out from a crate entangling the cleric having caught him unaware. The champion, ever ready for battle, leaps into action, ripping the cleric free and slashing wildly with his mighty blade at the appendage as it slips back into the crate..."
4. Give them something similar to hex/hunter's mark, that isn't a spell, and has a short rest recovery mechanic. Letting them designate a target and deal an additional d6 whenever they hit could go a long way to appeasing the apparent lack of power the champion has. Call it "Focused Strikes" or something.
5. Apparently advantage on initiative is both not too powerful and totally appropriate for low levels (*cough-rangerrevised-cough*) so give them advantage on initiative checks and call it "Battlefield Awareness" or something.
6. Buff remarkable athlete: you add your proficiency bonus as a bonus to all those checks, proficient or not. Basically gives the fighter limited expertise, just like the knowledge domain can for clerics. If expertised reliable stealth isn't broken, then expertised athletics/acrobatics isn't broken either. Or, give them straight advantage on those checks, and remove the bonus to the roll. Advantage would prevent cheese with rogue/bard MCs (improper wording could result in triple proficiency to stealth with expertise and remarkable athlete), and REALLY help out those heavy armor fighters when they need to be sneaky.
7. Hard to Kill: advantage on death saving throws, and +1 HP recovered when spending a hit die to regain hit points.
8. Improved Second Wind: You have advantage on the d10 roll to determine how many hit points are recovered when using second wind (or possibly they recover an additional d10).
9. Weapon Specialization: When you achieve 3rd level you choose a single weapon that your character specializes with, you deal an additional point of damage (maybe +2, not sure how the math would work out in the long run, trying to catch up the perceived shortcomings vs battlemaster) with the weapon, and have advantage on opportunity attacks made with the weapon. You may choose an additional weapon at 10th and 17th level.
10. Strength of Arms: when you are the target of a grapple or shove, and choose to resist with athletics, you may add the magical bonus of the weapon you are wielding to your roll, you further have advantage on checks to resist being disarmed.
11. Master-At-Arms: upon choosing this archetype at 3rd level, you may choose to take one of the following feats as a bonus feat: Alert, Athlete, Charger, Crossbow Expert, Defensive Duelist, Dual Wielder, Durable, Grappler, Great Weapon Master, Heavy Armor Master, Inspiring Leader, Mage Slayer, Martial Adept, Medium Armor Master, Mobile, Mounted Combat, Polearm Master, Resilient, Savage Attacker, Sentinel, Sharpshooter, Shield Master, Tavern Brawler, Tough, or War Caster.
Just some quick options off the top of my head to try and generate ideas you think would be useful to your enjoyment of the champion. Personally, I don't think it really needs to compete with the other archetypes all that closely, because there are ways to make greater use of improved crit in an MC build, but if you think it's weak or lacks flavor, man who cares, fix it the way you think it should be fixed. Some of what I suggested is PROBABLY too powerful, but hey, if you think the champion is lacking, maybe they won't seem too powerful to you.
Good luck in your games!
