First, some context - the 5MWD!Hopefully you are all familiar with the design arc that started with The Book of Nine Swords and runs through 4th edition into 5th edition. The 4th edition action types are at-will, encounter (refresh on short rests), and daily (refresh on extended rests). This structure was fundamental to the 4th edition design and targeted squarely at the 5-minute work day. The 5th edition rules carried those concepts forward while softening them somewhat as mechanics. (It's worth reading the 4th edition short and extended rest rules to better see how.)
I know what it is - where is it going?The 5-minute working day has been called out in some Level Up discussions as something that should be ended beyond all else. You might have also noticed the introduction of experimental rules in TCoE, that modify the 5th edition approach. First there is the shift of abilities that in the PHB might have been one or two uses per short rest, to being proficiency bonus per long rest. Secondly, there is a cute experiment with psionic dice where a character can refresh one die with a bonus action, and then that refresh itself refreshes with a short or long rest. (Psionic dice otherwise refresh on long rest.)
Perhaps I buy that it has been on D&D designers' minds - but why?If we want nice things, like strong powers and weak powers, and variation in the tempo of different classes (which changes how they feel in play) then one of the powerful design patterns available to us is currencies. Spell slots are a currency for paying for spells, for example. A strong spell costs a higher level slot, of which a character has fewer. So we have sorcery points, ki, superiority dice, bardic inspiration - all through the game are currencies. The currencies allow the game to have highly varied abilities for players to leverage the narrative with (more on leveraging the narrative below). However, none of those currencies matter if those slots, points, dice are topped-up all the time. And conversely, the game can be made more interesting if the currencies themselves refresh at different rates (albeit one does not want to get too baroque with that).
Leveraging the narrative?One of the most beautiful aspects of RPG game mechanics is that they give players defined ways in which they can have fiat (including stochastic or negotiated fiat) over the narrative. Casting a fireball lets the wizard write into the emerging story that there is a fireball, exploding right here, right now. That has proven extremely powerful as a concept. Players find it tremendously satisfying to say what happens. Of course you don't need rules for that, but rules regulate and validate each player's leverage over the narrative. One could say more about this, but hopefully you get the picture.
So what is the value in a great solution to rests again?Rest - regulated refresh of currencies - allow us to have nice things. Those things are the most fundamentally beautiful mechanics of RPG. Unfortunately, we do not yet have a great solution. You only have to have read these forums over the last few years to have seen all the problems with 5th editions rests. One whole edition of D&D targeted it squarely and foundered, and yet the game designers still knew that it was important enough to carry forward into the next edition, and are already at work on improved solutions.
But is this really that hard of a problem?Yes, it really is. Multiple editions - years of game design effort and playtesting - have still not solved it. It is the hard problem of RPG. The problem to solve could be put like this - how might we have rests that are easily applied by players, that reliably regulate the refresh rates of character abilities? This is something that needs to work as simply as possible, as appealingly as possible, for all D&D players, because all D&D players can benefit from solving this. A great solution will also have implications for other RPGs.
I'll rest here for a bitYour questions imply (at least) three concerns. What is the problem? Why is it worth solving? What solution - or direction for solutions - might emerge from this thread? I have said something about the first two: I'll take a break there, and do some other light activity for a bit before answering the third.