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.
Oh wow, I had no idea there was already a cover system in D&D! Thanks for letting me know. And it sounds like it's even more potent than what I suggested. I'll definitely have to read up on it more. And the readying an action feature sounds like a form of overwatch too, which is awesome.
As for Solasta, vaguely remember seeing it before, but I just looked it up and it does seem a little rough around the edges for my taste. But still, it's great to know that D&D already has some of these features covered (pun intended). Thanks for the info!