Legend of Ultima
Transforms into Mecha-Ultima!
It's true, Tidus and Jecht have been confirmed with scans of their Dissidia artwork along with a few screenshots displaying them against one another and it seems that the Arena Stadium where you must battle Braska's Final Aeon from FFX is also confirmed.
Official Link: http://www.jeux-france.com/news27869_tidus-et-jecht-dans-dissidia-final-fantasy.html
On a side note, it appears that all the FF Heroes are being drawn younger, just by comparing Tidus's alternative outfit to his FFX, though I have to say the Dissidia attire looks better on him, rather than his original outfit.
Official Link: http://www.jeux-france.com/news27869_tidus-et-jecht-dans-dissidia-final-fantasy.html
On a side note, it appears that all the FF Heroes are being drawn younger, just by comparing Tidus's alternative outfit to his FFX, though I have to say the Dissidia attire looks better on him, rather than his original outfit.