Ilbranteloth
Explorer
C'mon.... there's this room.... and this elephant....
...and these blind men... no, wait, different metaphor.
5e classes shake out into several different resource-recovery schemes. The prepped casters share one - many potent/flexible resources recovered on a long rest - and the other full casters are pretty close, that's like half the classes almost (Bard/Cleric/Druid/Sorcerer/Wizard), right there. Then there's the Warlock - fewer but still potent/flexible resources recovered on a short rest. The half-casters Paladin/Ranger/EK/AT - fewer, flexible but less potent resources recovered on a long rest. The Monk & Battlemaster - fewer, less potent/flexible resources recovered on a short rest. The Barbarian - one potent resource recovered on a long rest. And the Fighter - one potent resource, one minor, recovered on a short rest.
And, hiding in the shadows, wondering why everyone's always goofing off, is the Thief/Assassin.
So good. That's quite different than regaining hit points too frequently, or regaining long rest abilities too much, or people being always at their best each day when traveling overland, and...whatever other issues that have arisen.
My point is, there may be more than one solution that work together to make a whole. So knowing which specific problem(s) we're trying to resolve helps.
Next is to find out if everybody else agrees that the problem we're having is that the classes have asymmetrical recovery schemes.
So here's a different question on finding a solution. In AD&D, there were two primary resources - spellcasting and hit points. Originally, regaining hit points was way too slow to worry about. You needed magical healing for most situations, or you simply left the dungeon because finding a place to hole up for a week generally didn't work. So that meant that the only recovery time that was needed was a good night's rest.
Even when hit point recovery was faster, since it was just those two abilities, tied to a long rest (or something similar), it still wasn't much of an issue, right?
So for the short rest abilities, what's the expected recovery? Not what's implied in the rules, but what is wanted. Do we want a warlock to have to spread their few spell slots across several encounters, or should they have them available for most every encounter? What about Second Wind, or other classes' abilities?
It's possible that recovery for "short rest" abilities could be managed separately from "long rest" abilities, and those could also be different than hit points. For example, short rest abilities could recharge after 30 or 60 minutes of game time. Or perhaps a system where you can make a DC 30 check, with the DC reducing by 1 every minute to regain the use of the ability. That would allow you to potentially gain an extra use or two in the same encounter sometimes.
Long rest abilities could be regained during a good night's sleep, along with hit points. The amount of hit points could vary depending on how difficult you'd like the campaign to be.
You can see where part of my bias lies - finding a mechanical solution is fine with me, but I'd like it to be tied to the passage of time in the game world.