If they have their minds set on charging a separate admission go all out on the decorations and ride overlays in the park. Make it worth wild for adults to spend the extra money to enter. Trick or treating, extra characters, and a fireworks show is not worth the 40 extra dollars on top of an AP for most adults.
The most underutilized attraction is the Haunted Mansion during October. I think it's time to retire or delay HMH and bring Haunted Mansion up to its full potential with live actors. They could also do something amazing with Big Thunder Mountain Railroad.