CapnZapp
Legend
Not sure Morrus' friend share my interests...Not sure if he has responded yet, but this seems right up @CapnZapp's alley.
I should first say I'm envisioning an upgrade of 5E, not a supplementary toolset.
If I were to summarize a reply there wouldn't be any way around saying "more like 3E".
Not in the caster-martial balance or the insane NPC building workload, 5E is vastly superior in many (most!) areas. But in that there were many more ways a character could function in 3E. Most subclasses in 5E are just rehashing a skill proficiency there, advantage some check there. They always add breadth, never depth. Breadth meaning lost of new character concepts to play instead of your current character; depth meaning new subsystems that adds choice and variety to your current character.
From the player's viewpoint, what 5E lacks is more impactful decision points during all levels. Multi-classing helps a little, but isn't enough. Instead of just one major decision (subclass choice) there needs to be at least five, preferably ten (every other level). There's close to zero new mechanics subsystems since launch, and not very many to begin with.
You could retain broad compatibility with 5E while replacing advantage with a more granular system, re-adding back utility based magic pricing, and greatly upgrading the system's equivalent to a Monster Manual. The balance between melee and ranged characters probably need a tweak, too.
One advantage video game developers have is feedback data. If an ability isn't used enough in World of Warcraft, it gets a facelift or a rewrite. So take the 50% of abilities, feats and spells that are considered less effective and boost them, without touching the 50% that works reasonably well. This alone means a huge vitamin injection in that lots of formerly dismissed choices again become viable (to more players than those that don't care about efficiency).
Resist the urge to put a personal stamp on your edition. Don't add new stuff at the same time you're rebalancing the existing game!
As the DM I need much less out of an AD&D system than the players. Monsters designed with the actual abilities of player characters in mind. Utility-based magic pricing. These are the two big ones, and I've mentioned those already.