I thought of a pretty cool concept a while ago which would be pretty cool to have.
My idea was basically having combined skills for each job. How would they work? When 2 teammates of jobs compatible with the attacks have the amount of MP required, a screen pops up saying "Use Combined Skill?" and then they do it. They would also boost relationships. Here's an example:
Cannonball Slash (Warrior and Tank): Human Cannonball but the Warrior does a powerful Spin Slash when shot out.
Any more ideas?