There's one thing that bugs me about the December movie being pitched as the series finale... it's also the W crossover. If Toei does indeed plan to run W for a year, then they're going to want the crossover movie to promote W aggressively. They'll want screentime to be invested in the W characters doing things and an interaction with W's setting.
In Decade I noticed that in arcs where they had to move the plot forward somehow-- like, say, the Faiz arc that introduced Diend-- they didn't do a whole lot that was very interesting with the crossover. If they wanted to do a lot with the crossover, then any Decade plot development that had to happen in the arc was rushed through a bit (for instance, the Black RX/Black arc, where Tsukasa's first moment of real character development takes a clear back seat to HOLY **** GUYS WE GOT TETSUO KURATA).
I think this means there's a significant chance that Decade fans will be somewhat disappointed this December, because the movie seems likely to not actually be giving out all the answers we fans want to see. From Toei's point of view, it makes more sense to make sure it's a good W crossover first with elements of interest to Decade fans taking a backseat to that. We may get only a few new plot elements related to Decade revealed.
I'm not saying this will happen, mind-- I have no crystal ball, I can't read the future. But if the writers really wanted a stand-alone ending or a clear explanation of plot, they could've made time to include it in the last two episodes. That they didn't says a lot about their storytelling priorities and I don't see why the movie would suddenly change this (barring a different production staff).
I can easily see Decade ending up as the sort of franchise where a clear, logical explanation of the plot is only ever presented in gaiden material like a video game or sourcebook!