There's one actual feat not yet implemented in BG3 called Sharpshoot, which has something that works in a similar way to the homebrew abilities. It gives you the option of taking a -5 attack penalty while increasing damage by +10 if it hits.

Maybe the new abilities could be tied to a list of starting homebrew feats/abilities that you can pick from a list, and you could get trained to learn additional ones much later in the game. Going to quickly spitball ideas here, maybe archers could pick from one of three at the start of the game. (Okay, anyone could pick any ability, but for the sake of this explanation I'll just go with a list of what would be most relevant to them.)

- (Active Skill) Pinning Shot
- (Active Skill) Deadly Aim: Consumes your maximum movement points to enter a stance that adds a +5 attack modifier to your next ranged attack performed in the same turn. Activating this ability is considered a bonus action.
- (Passive) Steady Aim: The radius in which a ranged attack can be performed without disadvantage by being considered threatened or too close to an enemy is reduced.

(The latter two would be relevant to mages using spells that make attack rolls too.)