In 5e D&D, what would happen if someone house-ruled that proficiency added to damage rolls? How much of an impact would it have? In addition to the obvious, what likely unforeseen issues would have to be built around?

I'd only use it for melee attackers.

If it is all damage rolls, all the time: not much will hapen. All characters will be a bit more fragile. Characters who attack more frequently will benefit more from it than those who use other effects or single powerful attacks, obviously. Combats might be a bit faster, due to increased fragility.
A side effect is that hordes of minor foes (who by their nature attack often) will become more dangerous to higher level foes, but also thin out faster.

I don't think that it would be a massive distortion of the game. Those players who are affraid that their characters might come to harm, will (over-) emphasize the increased damage from minor foes, while those who want to hit their opponents hard and often will probably like it.

Quote from: Doom;843891I'd only use it for melee attackers.

Me too (or more likely, only for weapon attacks), but I have this odd ideas that fighters (and their half-brothers and step-siblings like Barbarians and Paladins) are supposed to do most of the actual fighting; that is a very different notion than the fools' ideal of all characters contributing equally to any combat situation.

If it also adds to monster damage, a lot of monsters with weaker multi-attack damage will have a massive buff.

If it is a blanket rule, rather than some player trying to aggrandize themselves, then the net result nay be not much of a change aside from combats going a little faster than the do now in 5e, which for us averages between 4-5 rounds I think. It also adds more math to the equation. One more step. Which may slow things down a fraction and negate the speeding up a fraction. YMMV.

Multi-attack monsters and characters will get that little extra oomph from attacks. It will be as if you gave everyone a magic weapon. Whats the point then?

This rule could make more sense if it was used only for the martial classes.

It's a house rule I used to use for the Fighter Types (Paladin, Barbarian, Ranger AND Fighter) in 3.x.  It would have a bigger impact (but not by much) in 5e, that said, it wouldn't be that massive compared to Magic.

Truth be told, I've been kicking it around for my games.

So, how would you address Two Weapon Fighting in that case?  Would the Bonus apply on both weapon attacks?  Or would it work on only one -typically main hand- attack?

And how would you address the Fighter weapon style, which allows you add you stat damage (typically Strength or Dex) to both weapons?

Because how it affects Two Weapon, would also affect Two Handed.