I think there will be a trailer with at least some new footage shown.
Also I feel as if a release window would likely be announced too, them simply saying "Coming Q2 2015" or "Coming May 2015".
I never expected it any time before Q1 2015, and the way I see it, the earliest possible release for it is May 15th 2015, if it was going to be released before March 2015 it would have been reflected in the profit forecast for the 2015 financial year, which ends March 2015, and May 2015 fits for a release for multiple reasons
1: It's just after the 2015 financial forecast explaining why it's not reflected in it, and would more likely be in the 2016 forecast, which is March 2015 to March 2016.
2: 15/05/2015 works as a release date that ties in with the namesake, ARRXIV was released on PS4 on 14/04/2014, so they may continue this release date format with XV.
3: May 2015 is about 1 year from E3 2014, giving SE one whole year worth of pre-release coverage starting E3 this year up until release, about 1 years worth of pre-release coverage is what FF games usually get before release once their release window is set.
FFXIII got a media blowout around the start of 2009, and it came out at December 2009, and a the western blowout continued until it's March 2010 release, that was just over one years worth of pre-release coverage in quick succession, XIII-2 's media blowout began January 2011 and it came out December 2011, about 12 months worth of pre-release coverage, with around 13 months worth including the western release, Type-0's media blowout also began January 2011 and went up until release in October 2011, 10 months worth, LR's media blowout began around the start of 2013 and went right up until it's JP release in November 2013 and until February 2014 for the west.
So if XV is shown this E3, then I believe it will be the begining of XV's media blowout, that means we have about 1 years worth of coverage from trailers, interviews, magazine scans etc, until it's release, and May 2015 fits that pretty snug, being just 11 months after E3 2014.
4: It falls just before E3 2015, allowing them to announce XVI without problem next year.
I think this E3 they will just announce that it's coming May 2015 or even just Q2 2015, if it was a 2014 game or Q1 2015 then we'd already know due to the projected forecast having an increase of profits factoring in its "supposed" release.
In addition to that, they also said in March that it was very far into development, and by September this year it would have been 3 years worth of full development, by March 2015 that seems to allow them enough time to finish it off and prepare it for shipping to make the May 2015 release.