The number on the cover.
Heritage and Background, merged. Like pick 2. This means that Background gets more heft. They both give a HD/HP, which does away with "max HP on HD at level 1". This also encompasses supernatural gifts.
Revise the Ranger.
Default to gritty rests. You can spend HD with medical or healing magic faster than overnight, recover
some HD and abilities overnight, but require an extended rest to regain most spells.
Fix the "back 10" problem (the last 10 levels of most classes, except 6-9th level spells, tend to be lackluster).
Give all classes 2 customization points, one of which happens at level 1. Like Fighting Style/Subclass for the Fighter, or Patron/Pact for the Warlock.
Get rid of Con-to-HP. Maybe con gives you additional healing when spending HD without magic, and makes overnight HD recovery better.
Add "when you are magically healed, you must also spend a HD to increase the healing". Without life force, magical healing can at most make someone stable (T3+ healing can sometimes bypass this).
Do a cleanup pass on bonus action economy.
Bake in a bit more Defender on Ranger/Paladin/Fighter/Barbarian. If a party wants more durability, adding a melee type should help more than a "Healer".
Do a "dip" balance analysis. Like how Paladins really want crits, Rogues want extra chances to hit/accuracy (and then crits), but don't get boosts to it in their own progression. Even if Paladins got it in the back 10, it would at least provide a path to getting it without MCing.
Revise the various "companion" builds, using lessons learned. Having a companion is one of the
two customization points such classes have.
Example: One of the 2 customization points for Ranger can be "Hunter's Mark" or "Beast Companion" or "Warden (4e ish)".
Bake in downtime to the core rules, abilities that let you do things during an extended rest.
Bake in NPC connections to charisma; basically, an NPC who is somehow "loyal" or "helpful" to you. Some backgrounds grant a pre-existing connection. Your limit is charisma bonus plus proficiency bonus, and they can only be gained in play, and can be lost even if you spend one of your slots on them. Think of it as an attunement limit; you can "maintain" cha+prof such relationships, and if you add more, the older ones sort of drift off (as happens in real life). These are not adventuring allies.
Rewrite XP. This is more of a DMG thing, but embrace using it for encounter building, which means throw out the entire large group multiplier thing. Encounter building both can be, and needs to be "add up points and compare".
Embrace tiers of play. Rework class abilities reflecting that. Iconic T3 abilities aren't "you hit a bit harder"; they should be there, but a class needs more in T3 than that.
T1: 1-5 (5 levels)
T2: 5-11 (7 levels)
T3: 11-17 (8 levels)
T4: 17-20, Epic (4 levels + Epic)
Yes, there is overlap. We can put major notes at:
T1: 3, 5
T2: 8, 11
T3: 14, 17
T4: 20, Epic
So two major "beats" at each tier.
Bring back OD&D based multiclassing, just because. Fighter/Wizard! This can even be via the subclass system, where your subclass is another main class.
Revise "alternative attack stat" mechanics.