Poked at this a month or two back - went in a similar direction in some respects, different direction in others:
Prodigy: +1 skill proficiency, +1 tool proficiency, +1 language(?), expertise in one skill you have proficiency in
Resolve: Once per long rest, when you fail a saving throw, you may reroll that saving throw. You must use the result of the new roll.
Persistent: As an action, you may spend a Hit Die to heal equal to a roll of that die. You may do this PB times per long rest.
(Tireless Stamina: Treat the effects of Exhaustion on you as one level lower? or just reduce Exhaustion level by 1 on short rests?)
First two are fairly standard; flexible, generically useful, relatively flavor-light. The other one or two (couldn't come up with a "mucking around with exhaustion levels" take I was satisfied with) were an attempt to lean into humans' real-life biological niche; we're persistence predators, built for endurance in long-distance travel. To my knowledge, that isn't actually a niche that's already being filled by a D&D race (Dwarves and Goliaths sit more in the space of hardy/resilient than high-stamina), so there's room to expand Humans into a slightly less generic space if one can come up with a satisfying mechanical expression of that endurance.
...obviously, the first potential issue with Persistent is whether the moves away from short-rest resources will affect how hit die-based healing works for all races/classes in "5.5"; the contrast with the current standard is most of the point of the ability, leaning into the "tireless" concept by making Humans less likely to need to take short rests (because they can get one of the major benefits of a short rest much quicker and more fluidly), which goes away if the current standard changes. The other potential issue is that putting different races on slightly different timetable might just wind up replicating or reinforcing the issues that already exist with different classes being on different timetables? I like the ability in a vacuum, but I could see it playing worse in practice.
Even if I'm not totally satisfied with the specific abilities I came up with, the general concept of stamina/tirelessness feels like a fruitful flavor-space to play in to come up with something that's actually interesting while still feeling distinctly Human.