I've switched to using the common modern D&D 3E / PF 1E / D&D 5E Attribute Modifiers, in my Original D&D / White Box FMAG implementation.  I wanted Attribute Scores and Modifiers to really matter. 

The player mechanics and monsters of White Box, are simple and easy to run.  I grasp it well.  Changing Modifiers was a simple modification.  Having an 18 give a +4 Bonus, allows a character to showcase their abilities more.

I also added Advantage / Disadvantage, from D&D 5E.

AD&D 1E combat system, with Lejendary Adventure character types, with X-Treme Dungeon Mastery style statistics, with on-the-fly adventure creation using the 1E DMG appendices A, B, C, and I, a brief section from Lejend Masters Lore on random events that seem especially apt for stealth and espionage type action, and the general make stuff up as you go system in Mythic RPG. With 1E encounter reaction table for NPCs you meet along the way with LA's quantified personalty traits.

I like the stat bonuses from Moldvay Basic so much I incorporated them into my 2nd ed house rules and one of my homebrew systems.

Been running B/X with my kids but added proficiencies, advantage, and roll high on d20 for everything from 5e.

Also trying to create my own system that is rules light.

When I was running 3E through 5E, I used an older school version of initiative (mostly, tweaked a little to fit).

The 5E advantage/disadvantage mechanic is a nifty lift in most d20-based games.

Now, however, I can't say that the things I'm using are that clear cut, except advantage/disadvantage. 

For example, my attributes are closer to BEMCI/RC than anything else, but not on an even curve.  I did take from that chart that it was better to keep it largely in the -3 to +3 mod range, with -3 and +3 being relatively rare.  That's something that I thought would have drastically improved 5E if it had been built in from the beginning.  Someone familiar with earlier D&D would see the roots of my attributes in that, but no one would consider it a direct lift.

In general, today I'm looking more at:

- What is the mechanic trying to accomplish?
- Why is it trying to accomplish that?
- How does that fit into the overall design of the game?

Then once I have the answer to those questions and know whether it is something I want for similar reasons, derive the mechanic to do so.

If there are automatic weapons, I always use the 2e Twilight: 2000 rules for 'spray & pray'.

Sure, this is one beauty of the OSR. I use advantage/disadvantage and d20-roll-over skills with Swords & Wizardry. I use OD&D side-based initiative with ... everything.

Our unholy OSR mashup combines character classes from Hyperborea, Oriental Adventures, Labyrinth Lord, and Swords and Wizardry with spells from these & Castles & Crusades, using the spell lists from the latter for appropriate classes.  We bring in house-ruled crits & fumbles.  Fighters & barbarians get the extra crit chance as per DCC.  From DCC we use spell failure, corruption, and deity disfavor, though we don't have the random spell effects.  We also allow halflings to dual wield and give dwarves the "sword & board" ability.  Gnomes get minor magical abilities as per C&C.  From S&W & Hyperborea we use the single saving throw.  From Hackmaster 4e we give characters HP "kickers" at first level.  So martial classes get a d20; semi-martial get d16; and non-martial get d14, plus their regular roll plus the CON bonus to each roll.   Only demi-humans can multi-class and they have class and level limit restrictions.  For hit dice, we generally use the highest available in the various game systems referenced above.  Ability score modifiers are standard B/X.  There are a few other house rules but these are the main combined features.

I use B/X morale a lot. Simple conditions to remember, and avoids the fight to the death syndrome.

I wrote a little game combining my favorite D&D (Moldvay's basic) with skills and feats that are a mix of 3e and 5e (proficiency bonuses) plus random casting (a la DCC) and at least a couple of things from 13th age and Pathfinder 2. But mostly Moldvay's basic; it includes morale, hirelings, etc. I like using morale and reaction regardless of which edition I'm using (even in 5e).

https://www.drivethrurpg.com/product/229046/Dark-Fantasy-Basic--Players-Guide

Now I'm basically trying to combine 5e and Shadow of the Demon Lord (plus Knave) to create my "minimalist 5e" game. It has not been easy, because I want to make it compatible with 5e modules. But I like what I've got so far.

(the main difference between the two is that DFB has -3/+3 attributes and most fatures are based on class; so a 10th level fighter has +10 to attack etc.; while my minimalist 5e does the opposite, leaving all bonuses to ability scores and doing away with skills, saves and even proficiency bonus).

One thing from modern editions I like to use in OS games is variable hit die; so a giant might have a d12 hit die, for example, giving it extra HP. I think it makes monsters slightly cooler and more threatening.