Actually, there IS a cover system in baseline D&D and it's more potent than what you describe. Half cover provides +2, three-quarters cover provides +5 and full cover means you can't be targeted at all. There's also something like overwatch in the player's handbook too in the form of readying an action. Which basically means you have your character at the ready to do a particular thing if a certain condition is met. Typically that means attacking an enemy if they come into range. Solasta: Crown of the Magister implements both.
