Mike and Karen

Here's how I would've included them: We focus on not only Andy in his new foster home, but Karen in a mental asylum, and like Sarah Connor in T2, she breaks out and rescues Andy. Meanwhile Mike would be trying to distance himself from the thing, but still talks to Andy over the phone because he can't with his mother. It ends with Andy and Kyle uniting with Karen and Mike. The four have the showdown at the factory.

Could've made a great 95-minute movie, instead of the 80-minute movie we got. Put all that stuff in there for 10 minutes' worth, 10 more minutes of Phil and Joanne, take away 5 current minutes. You don't really need that 5 minute block of Andy at school.

I think overall we would have gotten a similar film. Instead of a foster home setting, I can imagine Karen, Mike and Andy becoming a family and starting over in a new home. I think many of the film's elements would still be present: (The death of Miss Kettlewell, the factory showdown, etc) I know the mental institution is discussed a lot, but I feel like that idea was just a means to write out Karen rather than a character arc they were actually exploring.
In regards to why they weren't in the film, I don't think it had anything to do with money
necessarily, I just think that Don was going a
different way with the story.
Either way, it's fun to think of different scenarios.