Ok odd concept. But no levels. Instead at the end of every session each character gains 1 experience point (XP). And if a character has an inspiration at the end of the session and hasn’t spent it they gain an additional XP. Or they do something really cool so they gain another XP. Then you can spend XP to gain class and racial features.
The XP that you have to spend equals the level of the ability you want. If you would gain multiple abilities from a level, pick 1. Then if you want to multi class, it costs an extra XP for however many classes you have already, making multiclassing a little more difficult.
Then for every 10XP you spend your proficiency bonus increases by +1
Finally for every XP you spend to gain an ability or feature you have to spend that many days of downtime to unlock the ability or feature. This is you training and studying to master it.
Something cool, idk, do you think it’s stupid?

So like a point-buy system. Basically how Shadowrun and the WoD systems work (or at least the editions I played).

I think the biggest issue with it would be addressing how everything else that automatically improves in parallel with the individual levels would get handled. Like spell progression, Proficiency bonus, and all of those features that improve “when you reach Xth level in this class,” or “a number of [whatever] equal to [%] of your levels in this class.” For examples:

• If I save up my 1st level Rogue’s XP like a hoarder until I have 6 so I can buy them their second round of Expertise first off, does their Sneak Attack suddenly jump from 1d6 to 3d6?
• If I save my 1st level Barbarian’s XP until I have 20 and jump for Primal Champion immediately, that would also automatically double their PB (using your 10:1), do they also automatically get the unlimited Rages/long rest?
• If I save my 1st level Wizard’s XP until I could drop 18 all at once for Spell Mastery, how many Spells Known and Spell Slots do they have of what level? What about Cantrips?

Don’t get me wrong, I love Shadowrun and WoD. I honestly prefer them mechanically to D&D, so I like the idea of making a point buy system of character progression for D&D. I just think there’s still way more to figure out than how much you have explained so far.

As sposta notes, there can be issues with point but in D&D. Most point but are not only level-less, but also class-less. That can help avoid some of those issues, since every power you get is a la carte. Add a little to a weapon skill, buy a spell (usually I see buying each spell individually, not a whole level’s worth at once), buy some funky powers, and you’ve got the character you want. You can even make a legit gish.

You can make a very custom character. The down side is, it requires a bit of system mastery to avoid making a crappy character. And this edition has really worked to avoid that and let anyone just pick up and play.

Also, class and level is a pretty central concept to the game. If there was a new edition where they went point buy, it really wouldn’t feel like D&D anymore. It could be a really fun fantasy rpg, but it wouldn’t be D&D.

Maybe a way to fix the problems presented by IAmSposta is including a chart. I don’t particularly feel like making one right now, but it would make it particularly difficult to buy higher-level abilities, by making them more expensive depending on tier, such as only 1 XP per level of class ability between 1-4, 2 XP per level between 5-8, 3 XP per level between 9-12, 4 XP per level between level 13-16, and 5 XP per level between level 17-20, so Primal Champion would come out to a total of 60 XP.

Additionally, you would get class ability improvements based on the number of XP points spent on a given class. For example, in order to get 3d6 Sneak Attack, you would need to spend at least 6 XP, for the first four levels and the fifth.

Another way to do it is play the game that has it already built into the rule set instead of mashing D&D into something it is not!

HAHA! OK now that I got your ire up, some real suggestions:

Things that increase progressively like sneak attack would need to be purchased progressively. So you can purchase sneak attack at 1d6 for 1 pt then have to play another 2 pts for the 2d6 version, and another 3 pts for the 3d6 version.

I think the above could count for proficiency but proficiency covers so many things it should be expensive to increase. 

While we are changing things around, might as well drop stats from 1-20 to 1-10. Then each point actually has a bonus or penalty except 5 which is +0. Then when you want to increate you stat, you pay an amount equal to the current score - i.e. if you have 6 STR and want to make it 7, you pay 6.

Your spell system would have to be each spell individually with perhaps the chance to change the points around. Things taken for granted like cantrip increases might have to have a point cost for the increase in damage.

That's all for now.

I've read theory on these different kinds of progression tracks before, and for me the thing that hits the hardest boils down to how it feels to gain a level.

Tiny incremental bonuses happening at regular intervals don't give you a lot of satisfaction. Because they are tiny and incremental. Another session, another 1hp gained. Wrapping all those little things up into the big ball of dopamine that we call a level is just a much more satisfying gaming experience. Both the rarity of the progression in time and the non-trivial leap forward in power work together to give you a great sense of accomplishment.

Notice how all of Sposta's examples involve hoarding points for a big leap in effectiveness. Sure, it was to highlight difficulties the system would have, but it's also the first place our minds go. Because we want that feeling of a big payoff.

So intellectually I think it would be interesting to design a system like this for D&D, but emotionally, I'd never want to play it. Levels have remained a thing all this time because they are a really good, psychologically-proven system.

When it comes to point buy there is a little lingo that goes a long way towards making things easier: “X times Current” (#×C), and “X times New” (#×N). The first is an expression for a multiplier for the current number, the second for wherever the new number would be. For example, if you want to raise something from a 4 to a 5, and the XP cost is 2×C, then it would cost 8 because 2×4=8. If instead the cost were 1×N, then it would cost 5 because 1×5=5.

There are some great ideas here. Some of them are the same/similar to stuff I was coming back to mention, some inspired other ideas for me.

Wysp is right that the more useful something is to more things, the more it should cost. So specifically regarding the PB progression, instead of a 10:1 ration o would probably start at 30:1 and adjust up or down from there through playtesting.

I think Wysp’s also right about the scaling costs for scaling effects, which is another example of where that #×C/N would come in. (I would just scale things differently than they suggest.)

Another thing I both agree and disagree with Wysp about is the Ability scores. I agree that changing them would be better, I would just do it differently myself. I would completely do away with any Score->Bonus conversion entirely and adjust the scale slightly. I would make the floor -2, the ceiling 7, and do the point buy as follows:

• -2  ->  -1 = 1 XP
• -1  -> +0 = 2 XP
• +0 -> +7 = 3×N XP*

*So going from +0 to +1 would cost 3 XP, going from +2 to +3 would cost 9 XP, and going from +6 to +7 would cost 21 XP.

I also think MilestoGo_24 has a good idea about having things bracketed by Tier, but I would express it differently, make it a sliding scale with a Tier-Based XP Adjustment:

Feature Cost = 1 XP / Feature Level Adjusted as Follows:

 I might also adjust the XP Awarded / Session Tier-Based as well. 🤷‍♂️

 Xalthu’s observation regarding the classless nature of most point buy games is valid, so I would strongly consider grouping the class’ features into pools, so all of the Barbarian, Fighter, Monk, and  Rogue features would go into the “Martial Features Pool;” Artificer, Bard, Sorcerer, and Wizard would make the “Arcane Features Pool;” and then Cleric, Druid, Paladin, and Ranger would go into the “Divine Features Pool;” and then I would have Warlocks as a wildcard pool all on their own (or with Bloodhunter).

I would also probably swap everything from Spell Slots to Spell Points, that would help solve that problem a bit.

The real sticker is spells known/prepared.

In WoD & Shadowrun

That could possibly stay attached to Hit Dice?

Charging piecemeal would absolutely hose casters as compared to martials, and especially PCs with lots of casting and lots of features too. Like, charging piecemeal wouldn’t be unfair against Wizards because they ain’t got nothing else to spend on. But Bards would get shafted. No bueno.That is part of what makes Xalthu’s observation regarding the classless nature of most point buy games is valid.

The only other idea I had was this one, but it’s likely gonna need lots of work to balance, and it might not work at all. 🤷‍♂️
     Group the Class’ features into pools, so all of the Barbarian, Fighter, Monk, and  Rogue features would go into the “Martial Features Pool;” Artificer, Bard, Sorcerer, and Wizard would make the “Arcane Features Pool;” and then Cleric, Druid, Paladin, and Ranger would go into the “Divine Features Pool;” and then I would have Warlocks as a wildcard pool all on their own (or with Bloodhunter). 

Mechanically, that’s all I got.