D&D 5E The (possible) Core Rule in 5e

[I'm A Banana]

This new approach comes out of a single idea. At its heart, D&D isn't about rules. It's about participating in an exciting fantasy adventure. The rules are just the means to enable that to happen. They're not an end unto themselves. The reason most of us play is for the story that arises out of our games.

Player and DM
Player: You play a character in a fantasy world. Maybe your character is an elf, or a dwarf, or a knight, or a young wizard. Maybe your character is an intelligent donkey, or an old man who imagines buildings are actually great giants. Maybe your character is a doughty halfling, or a great treant. What your character is in this world is completely up to you!

Your main job is to pursue some goal of yours. Your character has a reason for being an adventurous person -- that reason might be greed, or love, or to protect the weak, or because you're a little crazy (or all of the above!). You might just want to become beautiful, help your friends, or kill a particular orc chief that killed your family.

DM: You run the fantasy world and all the adventure and conflict contained therein. Perhaps you create a world in which a dragon demands tribute from a small village, or a world in which a great army of orcs marches on civilization. Perhaps you know of a way for an ugly troll to become beautiful, or of a princess that needs rescuing from an evil witch. What adventure there is in this world is completely up to you!

Your main job is to place obstacles between the character and their goals. If they seek treasure, you place traps. If they seek to protect the weak, you place monsters that prey on the weak. If they are just a little crazy, you should have the characters they interact with react to their craziness. Keep the world interesting, and if the players start to feel bored, introduce a new complication into the world.

Player and DM Interaction
The simplest form of playing D&D is that the DM describes the sense, and the player or players describe how their character acts, and the DM describes what happens next. You repeat this process over and over again, weaving together a world and a character.

For instance, if the DM says "The town of Theropolis lives in terror of the great dragon Vilfangorn," you as a player might say, "I, brave Sir Strongarm, have come here to slay it and save the city!"

You might also say, "I, crafty thief Mad Mac McMahon, have come here to loot its treasure!"

Or "I, Farmer Gilbert, have long suffered under the dragon's reign of fear."

Or "I am the Lady of A Thousand Terrors, Bindlpyre, the Wicked Witch of the Sylvan Wood, and I have come to learn from this dragon!"

Or anything else your character may be in relation to that.

The DM, then, reacts to what you have said. He might say that Sir Strongarm has come to the village, but none of the villagers will tell him where the dragon is -- they are far too terrified. He might say that Farmer Gilbert has found a magical sword while plowing his fields that, legends say, was buried in the land by a famous dragon-slayer. He might say that Bindlpyre has come at a time when there are Dragon Slayers in the town, hoping to kill the dragon and take its gold for themselves.

This continues, with your character and the DM's world acting and reacting in concert. Your character tries to accomplish something, and the DM puts a roadblock or detail in your way, and then your character tries to overcome the problem in between your character and his or her goal.

Resolving Conflict: Sooner or later, your character might try something that the DM says has some chance of failure, or that might entail some risk. If you'd like to add tension to these scenes, you might want to roll a dice or flip a coin, or play rock-paper-scissors, to determine whether or not your character succeeds at the attempt.

For instance, if Sir Strongarm tries to persuade the people to open up about the where the dragon is located, the DM might say, "Okay, roll a six-sided dice. On an odd number, they remain tight-lipped, on an even number, they open up and tell you where the dragon's lair is."

For another example, if Lady Bindlpyre tries to kill off those troublesome dragon-slayers, and goes to challenge one to a duel in the streets, the DM might say, "Okay, flip a coin. Heads, you win the duel and leave that dragon-slayer writhing in the street. Tails, you fail the duel, and the dragon-slayer has you at his mercy."

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....and THAT is the core rule of 5e D&D.

Everything else -- bonuses, attack rolls, DC's, skills, races, classes, monsters, alignment, defenses, powers, XP....all of that is stuff you can add onto the core.

Because while almost every rule in D&D has its supporters and its detractors, in the end, when you think big, we're all just rolling dice to try and accomplish things during our adventures.

You're welcome, WotC.