@Atomicmickey - agree with you that that cave art could be something we see in the queue or boat ride. Given the whole culture and mythology of the Navi, there are opportunities for stunning artwork that provide backstory. Plus, Rhode loves details so we should be in for something quite special.
Joining in the 50th anniversary tangent...
DHS absolutely needs a Phase 3, but over the longer term. In fact, I'd rather wait for a 2025 opening and get a proper expansion than a phoned-in filler attraction opening 2021 that was done on a budget. Once Avatar, AK nights, TSL, and (especially) Star Wars open, AK and DHS both will have quality new additions that will
completely alter touring patterns at WDW (this fact is key for one park in particular). With these coming on line 2017 thru 2019, that will be enough to sustain buzz and crowds at those parks through the 50th.
The places where WDW will need to focus are MK and Epcot.
By the time 2021 rolls around at MK, New Fantasyland is going to be ancient news. At minimum, a completely revamped Tomorrowland (right down to every ride vehicle and facade) will be necessary, along with new parade(s) and firework show. Besides, I think it's fair to say that the park that is actually turning 50 years old should get something new to commemorate the event.
As far as Epcot goes, Disney is deluding itself if it thinks Frostrom and the new Soarin' are going to be enough to sustain that park through 2020 and beyond. MK will always attract crowds, especially during the 50th, and DHS and AK will have still-shining new rides and shows. Epcot is going to need major overhauls of at least 3 Future World pavilions (yes, 3 - pick any you want), a new fireworks show, and maybe even a new ride in World Showcase
just to keep up. For the past decade, Epcot's been able to survive not because because DHS and AK were viewed as "worse." Not only will the upcoming additions give DHS and AK formidable attraction lineups that will far outmatch Epcot's, they will also give those parks evening offerings that will rapidly eat into Epcot's food and beverage margins (pun!), starting later this year and only getting worse with time.
Without multiple major overhauls throughout the park, Epcot could well become the odd one out, a park people skip as they go to the shiny new things at AK and DHS. It desperately needs a DHS style re-boot in time for its 40th in 2022.