I'm skeptical of rumors suggesting there will be some new additions.
However, if there are, my money is on the volcano that was just created for the new "Trader Sam's" tiki bar at the Disneyland hotel.
http://www.youtube.com/watch?v=EKAIF9A9CDc&feature=youtube_gdata_player
As you can see, they use a window IDENTICAL to the windows at WDW's Tiki Room (in fact, isnt there already a volcano in those dioramas?)
I could imagine the room lights dimming and the volcano beginning to erupt during that short segment between the flowers singing and the tikis begin chanting (when the tikis on the ceiling begin banging drums). The thunderstorm would extinguish it.
Again, I doubt this will happen. But it would make the most sense since the effect was already designed for DL Hotel, and would fit the existing theater perfectly.