My two cents on this subject.
I don't think that Pandora opening will have any impact on MK attendance (nothing statistically significant anyway). That park is too strong, too established with a huge ride lineup and is capable of pulling all WDW fans in regardless of a person's interests in the Disney brand.
That being said, I think that it could have some effect on crowds at DHS, especially since 1/3 of the park will deep into construction at that time. It could even draw from Epcot insomuch as the flower and garden will either be ending or will have ended and there will be no festival to support it.
Where I really don't side with @djkidkas so much is regarding the direct impact on DAK attendance numbers itself. I think that there will be a relevant bump in attendance when this debuts, especially if RoL it up and running. To contrast a fully functioning nighttime lineup with Pandora and RoL against what was debuted last year is not even a fair comparison. This will be a brand new expansion that will be visually stunning during the day, and equally or more impressive at night. This will be a unique experience that will provide visuals and aesthetics on a scale not be found in any other park in WDW (or UNI for that matter). It won't just be two rides - and for me, the people that just rush through this park to do rides just don't get this park at all. But anyway, it will have walk-through areas, cast member interaction that will discuss facts about the animals, bioluminescence and history of the world itself, an eating location, a shop (of course) and a bar. This will then create a traffic pattern past the Nomad Lounge and Tiffins that will help draw more people there. Once that happens, the Nomad Lounge will become a hangout for a lot of people. And I think that RoL, with dining packages, will be a success (and something that they will most likely continue to improve). I just see attendance in this park increasing and staying, not simply peaking and then falling off.