I highly doubt Summoners are happening in this game; it would probably take up too much memory to make unique designs for each and...well, you get where I'm going. Remember, this is STILL a PSP game, and I'm sure they're already cutting down on plot and dialog as is (probably items too? Dissidia was cool for having such diverse items, but there really were TOO GOD DAMN MANY with a lot of redundancy in the system, especially for Trading Components. They could have easily cut down that item list in half and not suffered on gameplay much) to get more room for Character Models, and new attacks and such.
Though, as I've stated in the past, Yuna's X-2 form would fit into Dissidia rather well. Even has a logical EX Form...for all that I'm sure MOST OF US prefer to forget about it.
Lightning's EX Mode seems to be Gestalt Mode, not shockingly (...no pun intended), and Zantetsuken apparently has already been confirmed her EX Burst, with a Gestalt mode style Mini game!
Releasing Lightning next to Kain also suggests they're forgoing the Rivalry theme that Dissidia 1 had. I mean, why else would they have Kain next to Lightning?
*notices Kain's HP Attack is called "Gungnir"*
...cute, SE...really cute...
Also...
*Waves an "Orphan's Cradle Needs to be a Dissidia Level Damn it" flag*