I would LOVE it if the game was actually broken up into Levels 1-10 (as per many D&D adjacent games) in a PHB1 and then have WotC publish Levels 11-20 in a separate book.
When dividing the 5e Players Handbook into two separate books, a lower-tier (LT) Players Handbook and an upper-tier (UT) Players Handbook.
The lower tier can include a Master tier.
I advocate four-level tiers.
Levels: Tier
1-4:
Student (apprentice, page) ≈ Basic
5-8:
Professional (journeyer, squire, adventurer) ≈ Expert
9-12:
Master (guildmaster, knight)≈ 1e Name Level ≈ Champion
The LT Players Handbook has full Master tier, representing the 1e name levels, when the character attracts followers and builds an institution such as a fortress, wizard school, religious community, paladin order, thieves guild, and so on. The player should have freedom to design the legacy that the character builds.
Likely, many campaigns end with the Professional tier at level 8.
But there are campaigns that press on for a bit longer before retiring their characters, and the Master tier is for them to leave their mark in the world. When the players create new characters, they can members of the family previous characters, or pages in the fortress of the Knight, or apprentices in the school of the Wizard.
The Master tier enriches the D&D experience even for those who campaigns that only use the lower-tier Players Handbook.
The upper-tier Players Handbook is for a more superheroish D&D. A community of world-shakers, archwizards and other grandmasters. Epic is an option that the DMs Guide already touches on, and the upper-tiers can advance into it.
Levels: Tier
13-16:
Grandmaster (Archwizard, Noble/Lord/Lady) ≈ Master
17-20:
Legend ≈ Immortal
21-24:
Epic
The Grandmaster deals with other Grandmasters, whether as allies or rivals, striving to reshape the world. This is a superhero. Or a League of Superheroes battling against a League of Supervillains.
The characters step out of a medievalesque world into a realm of sorcery and magic, and power.
The Legend tier gains some form of immortality. Compare 4e. This can be an archfey, a lich, a demigod, or whatever. Some means of living forever.
The Epic tier acquires a "portfolio", a means to personally influence some aspect of the cosmos itself. "Boons" replace class features when leveling in the epic tier. A boon is a superpowerful feat. The boon may or may not relate to the class, depending on which boon the player chooses.
Same with MM1 & MM2 or DMG1 & DMG2.
Yeah. A lower-tier (LT) Monster Manual and an upper-tier (UT) Monster Manual.
I suspect the DMG need not divide, but can give all kinds of variant options for any levels.