Harry Pell of Wimbledon charged by the FA

As a minimum there should be a fine and some kind of ban. I imagine it comes under the heading of violent conduct which normally carries a three-game ban. Violent conduct bans can even be implemented when against inanimate objects. The FA usually view such incidents more harshly when supporters are involved due to the potential of causing serious crowd disturbances. A more extreme example was Cantona's leap into the crowd at Crystal Palace which carried a lengthy ban and criminal prosecution.
I don't think Pell's actions should be viewed as seriously as Cantona's (even though the provocation on Cantona was pretty severe), because he didn't actually leap in and personally assault someone, but it could fall under the umbrella of an assault charge if someone were to complain about the ball hitting them and they were injured.
I reckon it will end up as an FA fine, compatible with the earnings of a lower league player, a 3-5 match ban, and a warning as to future conduct. Part of the punishment may be suspended. If anyone was injured and complains, there might additionally be a look at criminal prosecution: this would possibly depend upon severity of injury.