I don't think the movie will add to the main plot in the manga. At best, it may use, or better explain, some of the more secondary plot points or technical descriptions of the world. Most likely will be a stand-alone story.
Anyway, poster looks interesting. It makes it seem like there is some link between Eclair (the Phoenix Priestess?) and Lucy. Perhaps some connection to Layla or the spirit world? A Phoenix's element is traditionally fire, so with Natsu, I'm betting there will be fire vs. fire combat. Betting Natsu will power up to defeat the big baddie by eating the special Phoenix flames. So we get another one-shot power up for Natsu.
Eclair's facial expression makes her appear a bit depressed about something, but also a bit angry, suggesting that she is either the main antagonist, or is assisting the main antagonist. But that she's ultimately a good person inside and is only doing it because she was hurt in some way.
Lastly, I took the contents of that website and ran it through Google Translate:
Spoiler for Google Translate:
March 22, "" priestess of Phoenix FAIRY TAIL "The Movie" official title of the movie the main visual and just completed has been lifted!
In the main this visual, unveiling the first time the figure of "eclair" mystery girl a guest character of the movie version. This character, has an important role has the stone of mystery called "stone Phoenix", are closely associated with the mystery of the work, together with members of the Fairy Tale, is a character we are involved in a conspiracy larger than ever before.
Also announced the details of the tickets were recorded with a comic drawn by Hiro Mashima special limited quantity will be released from (Sat.) "(Fairy Tale) + (plus) FAIRY TAIL The Movie" April 14!
"FAIRY TAIL + Movie" for benefits, luxury specification booklet of the same size as the Weekly Shonen Magazine. Special 30 page special edition comic is drawn, the prologue of the movie version and the main character has been drawn eclair. Director and cast interviews and messages such as interview, this was the rough draft version of the theater by Hiro Mashima and "sketch idea gallery", a luxury content aligned to the total page 48, before the movie, enjoy further benefits, even after the movie is.
Not too much detail, other than there is a conspiracy, and something about a "stone Phoenix."