I think I'm the only FF player who would prefer Lightning to Cloud. Clouds game was better, indeed. I really liked Lightning as a character though. (Seeing as she's most recent she is probably the most likely) Or Noctis would be pretty awesome even though that'll never happen.
My choice would be Lightning. She's the face of FF on the PS3 and her moveset can pull from the battle styles of the XIII games and Dissidia 012. Paradigm Shifting would allow her to be an interesting fighter.
Level 1 Super: Zantetsuken Lightning spins in a circle wielding Odin's blades and jumps slightly up, killing anyone who touches her. Can be comboed into.
Level 2 Super: Gestalt Mode Lightning summons Odin in Gestalt Mode and rides him. It would work like Fat Princess' Ride the Chicken. Short, but can score up to 4 kills.
Level 3 Super: Odin! Lightning summons Odin, and the player takes control of him. He works like Kratos' Lv 3 and can score up to 6 kills.
Alternate Costumes: Unlockable Costume- Her XIII-2 costume DLC- Her XIII-3 costume
For his move sets he would have a lot of range in term of her attacks. His  attacks would be as followed:
 is reserved for her gunblade Pressing  three times in a row will have her do his normal 5 hit slashing combo at the enemy (longer, but in the end it does the same damage as most 3 hit combos) Up  Launch- She sends the opponent upwards and follows them if button is held. Down  Smite- Flips and smashes enemy into the ground. Better in the air. Forward  Shoots 3 bullets from the gun and jumps forwards at them for a close range slash.
O changes her Paradigm and uses a Paradigm based attack. Up O- Commando (Increased Melee Damage) Down O- Ravager (Increased Magic Damage) Forward O- Medic (Increased Defense) O-Commando- Blitz- Spins in a circle, catching enemies and sending them upward. Ravager- Army of One- Guard-breaking Slashing combo at close range that knocks the enemy backwards Medic- Cheer- Raises offensive stats slightly (not to Commando or Ravager levels, but makes Melee more powerful than Ravager and Magic more powerful than Commando
/\ would be magic. Each Paradigm has a different set. Ravager /\ would be Thunder- a vertical burst of electricity some distance in front of her. Can be used thrice. Up /\ would be Blizzard- a shot of ice vertically that crystallizes when it hits the enemy and launches them upward. Down /\ would be Fire- fires multiple projectiles of fire at the opponent, stunning them. Forward /\ would be Aero- A burst of wind forward that, if it makes contact, pushes the enemy closer to Lightning.
Commando /\ would be Ruin- multiple bursts of energy just hitting them. Up /\ would be Swordstorm- Based off her moves in XIII-2 DLC, she surrounds herself with swords of air and jumps upward. Down /\ would be Ruinga- a larger blast forward that explodes and launches the enemy upward. Forward /\ would be... running out of ideas-
Medic /\ would be Cure- Slight AP increase. Multiple presses can combo. Up /\ would be Cura- Slightly greater AP increase. Requires more precise timing, but increases an ally's AP in 2 vs 2. Down /\ would be Curaga- Long charging spell, but has greater AP increase when used, like Cole's Sucker Punch but healing. Forward /\ would be... Evade?