I guess it would kinda be like an All-Star FF.

FFX battle system.
FFT Job System.
FFXII world.
It would have to have the Final Fantasy theme song and the Battle Song from Mystic Quest.

The characters: Bartz, Balthier, Fran, Penelo, Setzer, Yuna, Quina Quen, Quistis, and T. G. Cid.

It would have a mix of summons from various games, such as Diablos (Gravity), Bahamut (Non-elemental Blast), Shiva (Ice), Hades (Dark), Syldra (Water), Ixion (Lightning), Doom Train (Poison), Alexander (Holy), Lakshmi (Heal), Ifrit (Fire), Eden (Wind), Titan (Quake), each associated w/ a zodiac sign.

All these characters come together due to some snafu caused by Balthier when trying to use a new crystal to power his airship. It warps the characters from the other games into the world of FFXII & they have to stop the Trio of Garland, Ex-Death and Kefka from destroying the Universe.