I have a few rules that have made my plots a little more interesting:
1. Ghosts can join a household but cannot be permanently revived. They may come back briefly to finish some sort of task, but they must become ghosts again within a few days at most. Also, physical interaction with a ghost has a chance of putting a curse on a living Sim and causing them to join the afterlife, as well.
2. If you can become a PlantSim and you become Infected, you must also become a PlantSim. Mother prefers this.
3. The final tier of any adult aspiration may be cheated if desired. Child aspirations may be completely cheated when the child maxes out the relevant skill. (There are too many fun creative things for a kid to do for me to sit down and force them to draw the same five pictures every single time.)
4. Money and fame may be cheated IF starting a new family with a Sim who logically would be famous and/or rich already.
I think that's about it. These are really fun to read.