FFVI Worlds Collide

But of course, Terra's esper form would need a male equivalent for when a male character ends up with the Morph command. Take Terra's esper sprite and modify the sprite to have short hair with a male torso.

Or Morph could turn the user into a moogle, kupo.  


New commands can be created, but they have to be applied over existing commands.  This is because monster commands share the list that player commands do, and they don't play nice with menus.  It would take an exorbitant amount of adjustment to the battle engine to change a command beyond Magitek into something usable by the player, which is why it might be better to steer clear of that.

That said, Possess is a pretty useless command that is easily overwritten, and Health and Shock are easy to tweak.  There are a also few commands floating around this message board that can be swapped in for existing commands, such as one that works like Cover from FF4.
There are also some relic-upgrade commands that could be separated to give more variety to that list, such as Capture, Jump, GP Rain, Control, and X-Magic.