This question is entirely inspired by the following:
That question, however, happens to ask about a homebrew method of adding multiple ability modifiers to the bonus action attack of Two-Weapon Fighting and so, ultimately, has a homebrew solution. This question concerns non-homebrew methods of creating this scenario.
Two-Weapon Fighting states:
[...] You don't add your ability modifier to the damage of the bonus attack, unless that modifier is negative. [...]
But what happens when you would add more than one ability modifier to the damage roll? There are a few ways of doing this, and I'll list the ones I know of below:
The Oathbreaker Paladin's Aura of Hate feature:
[...] Starting at 7th level, the paladin, as well any fiends and undead within 10 feet of the paladin, gains a bonus to melee weapon damage rolls equal to the paladin's Charisma modifier [...]
The Ranger's Foe Slayer feature:
[...] Once on each of your turns, you can add your Wisdom modifier to the attack roll or the damage roll of an attack you make against one of your favored enemies. [...]
The Warlock's Lifedrinker Eldritch Invocation:
[...] When you hit a creature with your pact weapon, the creature takes extra necrotic damage equal to your Charisma modifier [...]
Note, that perhaps some of these aren't quite the same as the others, but what I'm asking remains the same: When a feature actually does cause you to add multiple different ability modifiers to an attack made as part of Two-Weapon Fighting which ones are not added to the damage roll?