For class design, I prioritize both customizability and balance. They are somewhat antithetical principles, but can be friendly to each other.
I am mostly happy with 5e class design. There is a wide viable range between lumping concepts into a single class and splitting concepts into separate classes. I feel 5e designs its classes in an appealing way somewhere between the two extremes.
My only "grievance" is, I need every class to decide its subclass archetype at level 1. Not level 3, nor 2. Even so, this can be minor nod, mechanically. For example, a magical Fighting Style for the level 1 Fighter can presage the decision to be an Eldritch Knight: perhaps a high elven Fighting style swapping heavy armor for Mage Armor, plus a Shillelagh-like cantrip that wields a longsword attack magically to substitute the casting ability instead of Strength. There can be more than one "eldritch" Fighting Style − maybe an other bypasses damage resistance.
I love feats and rely on them for character concept actualizing.
I am happy with the recent development of a "setting feat" at level 1. It allows the DM to incentivize the players investing in the themes of a particular setting. Compare Theros, Ravenloft, Strixhaven, etcetera.
I prefer every class to gain feats at the same levels: 4, 8, 12, 16, and I would have 20 (not 19), so that the PLAYER chooses which capstone they want for their character concept.
Ideally, I would like to separate out the combat-pillar feats, and silo them separately for these every fourth levels.
Then, have noncombat feats separate at different levels, to expand the concepts of the Background and the exploration and social pillars.
But the dual-use feats make such separation messy.
I prefer the tiers likewise increment by every fourth level, timing with the proficiency bonus improvement and capping with the feat choice:
Levels 1-4: Student Tier (Basic)
Levels 5-8: Professional Tier (Expert)
Levels 9-12: Founder Tier (Champion)
Levels 13-16: Master Tier (Master)
Levels 17-20: Legend Tier (Master wielding Wish spell etcetera)
Levels 21-24: Epic Tier (Lesser Immortal)
Levels 25-28: Cosmic Tier (Greater Immortal)
These tiers feel very different from each other, and inform the scope for designing an appropriate adventure.
Correlate "fame" with tier. So that the number of people who recognize (love or fear or both) is approximate: 10 ^ (level/2). Thus, levels 9 to 12 is a tier that has some influence over about 30,000 to 1,000,000 people. This is something like the population of a town or nation. At this tier, the character might "found" or reinvent a magic school, build a fortress, become a popular warrior hero like Beowulf, a prominent leader like a mayor, or so on, in similar scope.
At the Legend Tier, the challenges are planetary or planar in scope involve the populations of the entire planet recognizing the character.
The feat at level 20 (not 19) should be a highly desirable capstone. The player should think about its possibility even when building a level 1 character. As level 20 approaches, the player should drool in anticipation.
Different players want different things. So a capstone needs to be a choice among several excellent choices. In other words, the level-20 capstone needs to be a feat. Recently, the UA granted feats with level prerequisites. I find the level limits excellent, because the high level feats can be much more powerful than the lower level feats. The feats can organize by the every-fourth-level tiers, increasing power as appropriate to each tier. The level 20 feat is essentially the choice of an "epic boon", as found in the DMs Guide.
D&D 4e had the concept of the "Epic Destiny". There are many different ways to become immortal, and the player chooses how their character does it. Some achieve celestial apotheosis, some become a lich, etcetera. There are many interestingly diverse possibilities.
The level 20 feat for 50e should be a feat chosen from various feats, need a level 20 prereq, be equivalent to an epic boon in power − and also define how the character becomes an immortal.