That's what I think, if Terra is in Dissidia, then her Ex Mode will allow her to have the Morph ability to her Esper Form. Celes has no kind of special ability except magic.
In the trailer, at the time of 4:43 to 4:45, a figure, seen wearing fingerless gloves and had a short skirt, appears at the left side, behind the Warrior of Light, as seen in this pic below. Either it is Terra or Bartz.
The person who take this screenshot, use Photoshop to make the screenshot of the pic more brighter and clearer.
In another certain point in the trailer, where Sephiroth fought against Squall in a one on one swordfight at the time of 4:00 to 4:05 secs, and begun to slash his opponent at 4:06 to 4:07 secs, it seems that Sephiroth is not attacking Squall but a certain individual. Some fans claim that they saw a blurred figure, seemingly evading Sephiroth's attack in a fast motion and appears below his right arm, of what appears to be a spiky blonde hair (Cloud?), at a split second. Pausing and step by step slow motioning the part, results in seeing it. I'll try to post the pic later.