Athletics feat
The existing abilities that the feat grants are rather nice: You don't want to add much to it or it will become too powerful.
I'd suggest not expanding Athletics skill too much: its already head and shoulders above Acrobatics.
Perhaps a small speed boost when you take the Dash action?
What I mean is that the feat doesn't even come close to rather common spells. In a game where you can spider climb, levitate and even misty step, mundane increases of regular movement modes simply don't cut it.
As a power gamer you simply never take a feat like this, you go straight for the abilities that really make a difference.
To justify its cost, a movement-related feat can't exist in isolation. Yes, its modest benefits might be realistic, but D&D offers so much more. It comes across as way more effective to focus your feat purchases on things that magic doesn't offer as cheaply.
As for Athletics the skill, I probably was confused when I wrote that. Yes, I had the imbalance between Athletics and Acobatics in mind, I just confused which was the better one. In d20 both had their applications, now one has been robbed of most of its utility. But lets leave it at that, I really did not mean to bring a skill discussion into this feats workshop.
So back to Athletics the feat
As for Athlete, combine it with Grappler.
When I re-did some feats, I combined Grappler with Tavern Brawler
That's certainly one option (combining two weak feats into one decent one)
Athlete - Maybe increase your dash speed? Advantage on one strength or dexterity check per short rest? (though that adds more accounting fiddliness)
Getting back to this, my primary thought is to take Athletics as far as possible without completely entering the supernatural.
Analyzing the RAW feat in detail, we have:
* +1 makes it a half-feat
* stand up from prone costs 5 ft instead of half speed.
* climbing doesn't halve your speed (errataed into climbing doesn’t cost you extra movement, which actually means you become immune to difficult terrain while climbing)
* make a running jump after 5 ft instead of 10 ft
First off, benefits #2 and #4 come across as incredibly bean-counting and petty. This is a game where you fly or teleport across battlefields. This is a game where monks can flying kick your face in from a hundred yards away. #4 is actually kind of broken, together with the Jump spell, since you're still restricted by your Speed.
Because you can't run your speed and then make a jump, the game suffers a lot from the artificial segmentation that is combat rounds. (If the chasm is right at your feet, or starts too far away, you simply can't jump across it this round. In the first case because only movement this round counts towards making a running leap. In the second because the jump can't exceed your Speed this round.)
I would very much see if we can't burst all these petty limits before we look into combining it with other feats.
How about...
Athletics feat
- stand up from prone costs 5 ft instead of half speed.
- you gain "Swash Speed 30". Swash Speed lets you move freely using ropes, vines, chandeliers, handrails, ship rigging, the shoulders of peasant crowds, anything solid really. You can move in all directions, although climbing straight up still costs you 2 ft for each feet of movement. Swash Speed still require you to adhere to gravity and friction. You must move on top of items - you cannot use Swash Speed to climb on a ceiling upside-down, sit like a flyon a wall or ascend smooth sheets of ice. Unlike wuxia masters, you still have weight - you cannot run on thin glass, lily blades, paper walls or empty air.
- you may take the new Leap action once per round
Also change the feeble Jump spell:
Jump
You touch a creature. The creature may take the new Leap action once per round until the spell ends.
Leap (action)
You make a mighty leap, jumping up to your Speed lengthswise and half your Speed heightswise. You must land on something solid at the end of your jump, or you fall.
The regular rules allow for a Strength 15 fighter making a long jump of up to 15 feet and only after 10 feet of run-up, and a high jump of 3+2=5 feet, again only after 10 feet of run-up.
This is pathetic, and never in my history of DMing D&D have we ever used these rules.
Not only are they fiddly, they are very weak sauce indeed. Not only are they weaksauce, they're almost impossible to actually use in practice, since the restriction of a jump having to take place entirely within a single round's worth of movement (60 feet) isn't lifted.
No wonder players simply say "you can't jump as a regular D&D hero" and go straight for the Slippers of Spider-Climbing, the Fly potions and the Shadow Monk's Shadowstep!
With this feat you simply jump up to 30 feet long and 15 feet high.
No need to check how much movement you've used before or after the leap.
This brings the ability much more in line with what the game's other options already offer.
Zapp
PS. Note how I haven't (yet) included the +1 to Strength/Dex. If y'all feel Swash Speed™ plus Real Leap™

is attractive enough on its own, there's no need to add it back.