1. Consequences
The first series was very good at showing how much trouble the gang had covering up the deaths of everyone they killed, but since then the characters seem to have had quite an easy time hiding the bodies with no one getting suspicious. While this was a good idea in the TV show, as it would get rather repetitive if after killing every probation worker they had to spend the next few episodes trying to make sure no-one found them, I think a movie would be the perfect opportunity to show the police becoming suspicious of all the disappearances around the gang, even if this was just a sub-plot that was resolved by the end of the movie. This would help ground the show a bit more, as often events seem to be happening in their own little bubble with no connection to the outside world, and it would be quite nice for the gang to finally have to take responsibility for their actions. Or at least consider taking responsibility for their actions. Also, on a side note, is it just me, or do the gang seem to bury bodies in the exact same spot in the woods every time they go there?