I think if there is a third one, it definitely need to have more Beetlejuice / more comedy from him and perhaps a conclusion or redemption arc to his character. They need to move away from the town from the first movie, but could still included the next chapter of the Deetz family.

There should be a bigger threat, a reason for Beetlejuice to be needed once again. You kinda feel sorry for him that people are using him then just get rid of him when they are done, even though in the first movie he did have his own motives. It was hard to tell his motive in the second movie other than he has been wanting Lydia Deetz for 30 years but couldnt get back?

Perhaps the third movie could be a wild apocalytpic trip where we see more of the spirit world, rather than just the office environment, or a merging of the spirit realm crashing into reality. Beetlejuices wife should be back with another big bad.