-Decade and W will have very little-to-nothing to do with DaiShocker. It seems Showa era stuff will be used in this movie, so I think all this is saying is that the plot of this movie is something entirely new, possibly borrowing characters from the DaiShocker story.
-Dr. Shinigami will revive. This will have something to do with Double.
This revived Shinigami may infact be some kind of fake. f I had to guess, maybe someone using a Gaia Memory to access his powers or something.
-Narutaki will finally reveal his power. He is able to Kaijin Ride into Showa Era Kaijin, just like Decade is able to Kamen Rider into Hesei Era Riders.
-The movie, as a double feature, will have both Decade and Double starring in their respective stories, though there is supposed to be a small amount of interconnectivity between these two movies. The connection involve Decade appearing in the Double movie, but not much more is known besides that.
-As for the Decade story, people should expect to have a good amount of their questions answered, but of course, not all of them, because this is Hesei Era Kamen Rider. If there weren't questions left over, it wouldn't fit being a hesei-era finale.
-Kivaara is in fact Natsumi's "Meron":
http://en.wikipedia.org/wiki/Meron
It's complicated stuff, but it seems to deal with a tunnel connecting two different points. Perhaps regular Natsumi and Nega Natsumi?
-Yuuske seems to have been lying about something.
-Tackle shows up.