Here are the house rules I applied to the 1981 Basic D&D rules. Ran game, modded to get what I wanted. Results: 1981 House Rules

Ability Mods: We use the 3e/4e convention (+1 or -1 per 2 above/below 10)

Saving Throws: These are ability checks, DC determined by DM

Attacks: Ability check, plus a class based bonus
Fighter: +1 every odd level
Cleric/Thief: +1 at level 3, 6, 9, 12, 15, 18
Magic-User: No bonus

Thief Abilities: These work a bit differently.
Open Locks, Find or Remove Traps, Pick Pockets, Hide in Shadows are all things only thieves can do. Hide in shadows is literally that – hide in situations where other people can’t. The thief makes a check with a bonus equal to the % listed on page B8 divided by 5.
Move Silently, Climb Sheer Surfaces, Hear Noise are all things that anyone can try. The rogue has advantage when he tries any of them.

Advantage: Roll an extra d20, take the highest result. If you get advantage more than once, take an extra die.

Disadvantage: Roll an extra die, but take the lowest. If you have advantage, disadvantage zaps one die per instance of disadvantage.

Hit Points: Upgrade everyone by one die type, you get maximum hit points at level 1 + half your Constitution score.

Hit Dice: You can use your hit dice to heal. You get hit dice equal to your total HD, spend them when resting, each die gets a bonus equal to your Con modifier.

Dwarves: Increase class’s hit die by one size. Infravision 60 feet. Can use Find or Remove Traps in underground locations.

Elves: Can alternate between magic-user and any other class, have infravision 60 feet.

Halflings: Can Hide in Shadows as a thief, get a +2 bonus to AC, but use an HD one step smaller and can’t use two-handed weapons.

Humans: Gain a +1 bonus to any two stats, or +2 to one stat (maximum 18).

Weapons: d4, d6, d8, or d12, class access based on die size
Fighter: All
Cleric: d6, bludgeoning d8
Thief: d6
Magic-User: d4
https://t.co/vVyhUGAb0F

— Mike Mearls (@mikemearls) April 17, 2015

The Basic D&D mods answered the question in 2012 for me – what would I add to a very simple D&D ruleset to make myself happy?

— Mike Mearls (@mikemearls) April 17, 2015

@ChrisPerkinsDnD Mr. Perkins, how would you suggest someone with a genuine passion for D&D and Writing get involved with WotC??

— JB Little (@AnOsvaldo257) April 20, 2015

I know that @christulach is always on the hunt for OP adventure designers who know the rules and know how to write. https://t.co/vLNXY475DL

— Christopher Perkins (@ChrisPerkinsDnD) April 20, 2015

@ChrisPerkinsDnD Worldbuilding: Island where wood elf druids & rangers guard portal to Arborea. What one deity to the elves worship?

— Eric Radej (@yetibex) April 19, 2015

Rillifane Rallathil, elven god of nature, is an obvious choice. Fenmarel Mestarine, if the guardians are outcasts. https://t.co/CD6UQYFizW

— Christopher Perkins (@ChrisPerkinsDnD) April 19, 2015

@ChrisPerkinsDnD What if they're not in the Realms? ;P

— Mike Draco (@digitaldraco) April 19, 2015

Then use Asmodeus. The elves are pagan outcasts helping the infernal legions enslave the wild denizens of Arborea. https://t.co/TKwOiIonhx

— Christopher Perkins (@ChrisPerkinsDnD) April 19, 2015

@ChrisPerkinsDnD our group is creating Dragonborn PC's for The Dragon Queen adventure. What should we call our gathering? (Tiamat Colors)

— RokImmortal (@RokImmortal) April 19, 2015

The Scales of Justice! 😍 https://t.co/4t3wEXwiCY

— Christopher Perkins (@ChrisPerkinsDnD) April 19, 2015

@GWJRabbitrules check? 6 skirmishers split into two groups of two by casualties. Are any “isolated?” Do they retain the same initiative? retain initiative, not isolated

— Mike Mearls (@mikemearls) April 24, 2015