However, for those of us who like some of the more simulationist aspects of D&D combat, some of those discarded rules are missed. Combat is chaotic. It should be hard to shoot an arrow at an opponent while avoiding hitting your allies when they are engaged in hand to hand combat.
Also, for those of us who like a little old school flavor, not only do we want it hard to shoot into combat, there should be a chance that you accidentally shoot your friend in the back!
Today, it came to me in a flash as I was looking at the Hackmaster rules. In Hackmaster, it's very dangerous to fire a missile weapon into an engaged combat. You are just as likely to hit friends as you are enemies. In Hackmaster, if you miss your intended target, your attack then targets every other creature in the vicinity until it hit one of them (or misses them all)
It occurred to me that D&D 5e has the perfect solution for this mechanic.
Disadvantage (but with a twist)!
Taking a page from the Hackmaster rules, here's my idea.
If you are an archer (or spell shooter) and you fire at an enemy target engaged with allies in melee, you roll Disadvantage on your attack.
Here's the kicker: If your attack misses, you then take the higher of the two rolls of the Disadvantage (as if it were Advantage), and use that higher roll as the attack against other creatures within 5' [adjacent square] of the target (starting with the one who is the most in the "line of fire" and working around to the others in the area in some ordered manner like clockwise). If the higher of the two dice in the Disadvantage roll penetrates the AC of one of the nearby creatures (taken in order), that creature is the resulting target. If the higher roll is not enough to beat the AC of any of the nearby creatures, the shot is a true miss. Keep in mind that the new target could also be another enemy which adds a bit of random fun to the house rule.
EDIT: +Dave Sherohman makes a good point about this penalizing high skill archers too much. To clarify this, the "miss" roll does not use the PC's proficiency. I.E. - An AC 15 would only be struck by a "15" on the miss die without any bonuses from the PC's skill or buffs. Thanks for the comment, Dave!
Also, several people have pointed out that the DMG suggests using cover rules which work fine for that, but I actually think this solution is more elegant in that you don't have to argue about what is 1/2 cover versus 3/4 cover and you don't have worry about the fiddly calculations about if the to-hit roll hit would have hit the cover or not. Just look at the second die roll and see what AC would be hit on the die.
In my next campaign, I intend to go much more old school with 5e. Resource management, less healing, more gritty realism and grim danger. I think this mechanic will really make the players think more carefully about their missile and melee tactics, and I'm looking forward to see how it actually turns out in play.