I would say add the NBC overlay to the Haunted Mansion in WDW, but it was already supposed to happen and didn't, so I don't see it happening anytime soon, and just having it in DLR and TDL makes it more special.
Imagine an Easter overlay of Main Street. That could be... Interesting...
I wouldn't mind seeing more decorations around all the parks. Seems MK was the only one with Halloween decorations when I went. Actually, it was the only one...