I know that some people have proposed that if it would have hit the target before the +2 bump to AC and now misses that it hits the blocking creature.
Misses: If the “to hit” die roll indicates a miss, roll 1d6 and 1d8. The d6 indicates the distance in feet the missile was off target. (If the target was large, simply compute the distance from the appropriate portion of the target, ie. The character aims at a section of the floor which is 1’ square, and miss distance is measured from the appropriate edge as explained below.) The d8 indicates the direction in which the distance in feet of the miss is measured:
1 = long right
2 = right
3 = short right
4 = short (before)
5 = short left
6 = left
7 = long left
8 = long (over)
At short range you may optionally use d4 to determine distance off target, but then use d8 for long range distance determination. If the missile is hurled at a plane such as a wall, read long as high, short as low, measuring up the wall and then along the ceiling or down and then along the floor.
That's kind of what I was looking for. I wondered if the DMG had put that out as an alternate rule.
But assuming there's no one in the way, there's no penalty for firing into combat, right?
Yes, this optional rule is in the DMG on page 272: Hitting Cover. Personally, I plan to implement it in my game.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.