I don't think there is anything inheritantly wrong with having a epilogue Movie, however the issue is with how it's done, first off the main story has to be completed in the manga, we can't have the enemy pop up in the epilogue, and second the epilogue has to fit in well with the actual content of the manga. For example if it ends up as a love story between Lissana and Natsu, then the movie would need to use the actual manga as the main source of the pairing, not make up some crap from before the manga started and then dismiss the bond between Lucy and Natsu as Natsu seeing Lissana in her or something equally lame, that would have to be convincely explained. And most importantly it has to be written by Mashima not the anime producers and then trying to sell it as a piece of work created by him, (I still think the Last movie should have been investigetd for misleading advertising as it was advertised as being a work of Kishimoto and not something written bysomeone totally unrelated.)
As for this weeks chapter, the sad part is I'm not even remotely surprised by the outcome. What was the whole point of God whatshis face? He changed sides, turned out to be a fake dragonslayer, and owned. from all the pointless hype buildups in this manga this might just be the most pointless and dissapointing one to date.
See, this is my whole problem with this arc and the upcoming movie. It feels like it's been a mess lately, with how Mashima is trying to introduce too many plots at once and rushing it altogether. And then, just as Acno shows up, we're getting new info on the movie which previously had a teaser image of a half-dragonized Natsu. Considering the closest thing that can defeat Acno is a dragon, I really feel we'll have a "The Last" scenario where Mashima is trying to end the manga at the same time the movie comes out.
Worst of all, I feel that Mashima will downright troll us and skip the final battle against Acnologia straight to the epilogue for the movie. The movie will be about the final battle against Acnologia. At least, that's what I'm thinking.