The more I played 5E, the more I appreciated the classics.
What turned me off to the 2023 version of 5E is the rampant power gaming and cheating (and a lot of official 5E content does this) of expansion books. Games like Level Up A5E do an excellent job of establishing a sane baseline, but once you bring in any legacy book, the balance gets thrown out the window.
5E is to D&D what Modern Star Wars is to Star Wars. It is a 'Homer's Car' version of the original game, with so many great ideas piled on the game that it is an unrecognizable mess. The more books you add to get it playing how you want it to, the worse it gets, and the more books you need to fix the problems introduced in the books you added.
You are soon forced to baseline by returning to the original 3 books or a new version like Level Up A5E or Tales of the Valiant. Still, even going back to these as baselines put a band-aid on the problem; once you add expansion books, you are back in the same boat. The best way this works is to throw out expansion books and start fresh.
5E isn't a game but a kitchen junk drawer of ideas people call a game.
Many of the ideas they introduced in 5E (bounded accuracy) cause more problems than they solve, and they aren't as huge of improvements as people make them out to be. Even advantage/disadvantage isn't that revolutionary a mechanic since it is only +/- 3. Rolling one die and adding or subtracting three is faster and simpler than rolling two dice; this is grade school math. The A/D system prevents you from using a -6 or lower modifier for complex challenges and ends up being a one-size-fits-all solution for problems that require nuance and judgment.
So I returned to Dungeon Crawl Classics and rediscovered that Appendix N goodness.
I needed to clean the palette, rediscover fantasy gaming, and get out of this "gimme something every level" slot-machine addiction feedback loop of modern game design. 5E is designed like a mobile game; it gives me something at every level, and it shows. Even feedback from 5E players I see online who play other games (like DCC or C&C) get that instant dopamine withdrawal symptom when they play other games, "This game doesn't give me things at every level like they do in 5E!"
75% of the time, how you make 5E players happy when switching to other games is to implement a feat system at level one and every odd level after. They will be happy even if you "let them make the feats up" or adapt 5E or 3.5E feats. C&C has a feat system (called advantages), and DCC even mentions implementing a feat system in the game if you want to (page 447). You can buy books of 5E feats or PDFs of all the 3.5E feats that are free online - use those for inspiration for both feats and subclass abilities.
If you allow advantage/disadvantage, all that is left are subclasses. I could mod subclasses into DCC or any OSR game by starting them at 2nd level and allowing players to invent a subclass and gain a feature every even level. Make it a 15% benefit at one thing, and you are fine. I am a life order cleric; I get +1 per die of healing. If it is a 30% benefit, give it a duration or number of times a day equal to the level limitation. And subclass features can stack and double up if you don't want too many of them. DCC makes things easy by allowing a shift up the die chain for some abilities.
Most players can imagine a minor subclass benefit they would like to have and be able to balance it themselves. If it is broken, adjust it. If it is broken and can't be fixed, that character is a one-of-a-kind, and never allow it again. If it is too weak at higher levels, give it a buff.
Feats can be taken by anyone and are more general. Subclass features are limited to classes and often focus on improving the class abilities. Want to replace a feat with a "race or background ability feat?" Go ahead if your character would qualify.
This is precisely the "game design" that Gary Gygax and everyone else did around the table as they developed the game. We did this in the 1980s! Designing the game as you play to be "your game" is just as Appendix N as anything in a rulebook.
There. That is the imitation secret sauce of 5E. A feat at every odd level. Gain one subclass feature every even level. Every 4 levels, raise two ability scores by a point. People rarely play above level 12, so the number of "things to track" won't be very high.
Most importantly, everyone is a game designer.
This is my problem with 5E. I could play with five shelves of broken, low-content, consumerist game books and waste most of my time searching for the one or two options I want. Or, I could use a simple game 'built to mod' like a DCC or C&C, and play that while modding it to precisely what I want. With the 5E imitation secret sauce, any of these can play like 5E and give that dopamine hit.
Most importantly, I trust myself to balance and create options far more than the designers at Wizards or a dozen 3rd party companies interested in selling books built for power gamers.