Aaarrggghhhhh!!!!!!! I have been writing again and again parts of this on this thread, sometimes in direct reply to you.
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The best class in 5e for character development is the Warlock because you get to choose how you level up, by picking invocations. Two warlocks are not the same as each other (and it would be better if you replaced roughly half the invocations with good ones). Second best is either the sorcerer (your spells are determined with each level) or the artificer (what did you spend your time learning how to create)? Most of the classes with spells have either the same spell list or at worse can switch books.
I want other classes to be able to do this; once you have selected a subclass to not spend the rest of your time just levelling up as a cookie cutter member of that subclass unless you abandon your class entirely to multiclass. And OneD&D feats are better - partly because the feats aren't so in competition with ASIs and partly because they are much better balanced meaning there are more good choices.
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The fact that levelling is passive only makes it push the story more because the players and characters have less control over it and it forces them more down a predetermined path. And you have to write your settings such that levelling with the inherent vast disparity in power is a part of them.
Now, I'll give you a mechanic that to me is one that encourages story from Apocalypse World (from memory)
When life becomes untenable
choose 1 [exclusive so each can only be picked once]:
- Die, permanently
- Come back with -1 Hard
- Come back with +1 Weird
- Come back with a different playbook [class]
(The Apocalypse World stats are Hard, Hot, Sharp, Cool, and Weird and a +1 modifier is significant).
Do I think that this sort of mechanic would work in D&D where coming back after death was fairly cheap but had limited uses and a player chosen long term consequence (for example replacing your subclass with one from the god or patron that brought you back?) I certainly think I'd like to see something inspired by this mechanic rather than no consequences at all other than diamond and spell slot cost.
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5e has a tool for storytelling that I don't think is pushed enough although I've used to good effect (having stolen the idea from Apocalypse World) - the subclass change. The Paladin of Valour who loses the faith can become a Paladin of Redemption, of Glory, of Conquest, or even an Oathbreaker - or a Warlock can change patrons. This is a very powerful tool and should be used sparingly (I've even turned a battlemaster into an echo night for reasons)
This also ties into my pet hate about 5e's storytelling - character growth is on rails. Once you have your subclass at level 3 you've very few important choices left to make; you're almost certainly taking an ASI in your primary stat at 4 or 8 and most of the +1 feats just aren't that good. Who most characters are mechanically at level 11 is almost entirely predetermined from who they were at level 3 unless they multiclass. (This is a key reason I like the OneD&D feat changes).
The other biiiig thing is a lack of consequences for combat. No Rust Monsters. No wounds. No scars. Either you die or you're fine.
I'd also give a lot of monsters a "short rest action" in their statblocks - so if the PCs stop they can recover their wound and are now fighting slightly empowered versions of their foes. And more weight on what you can do with loot.
Again I think I've been pretty clear in what I want and how it can tie into 5e's existing mechanics. And you'll note that most of what I want isn't additional character power like feats. It's allowing characters to change to better reflect who they are and what has happened to them. "Emotional growth feats" are the most ham-fisted way I can imagine to do this.
