I tried implementing this for Fergus in FH1 and it didn't pan out. I've figured out the problems and am adding it to the sequel.

These bonuses will work differently for every character. Here are examples of what I have planned so far:

Maddy: 100% health enables combo strikes. As soon as her health goes down, they are disabled. This will make her useful in a party situation.

Fergus: under 20% health enables combo strikes. This is the last stand type condition that I attempted before. Pretty useful for boss fights.

Haddy: 100% MP enables double spell effects.

Meera: under 20% MP enables double spell effects.

You get the idea. Still have to figure out other ones for the rest of the characters. In the third game, I will go an extra step further and have enemy battle bonuses that can work both in your favor and against you, depending on who's in your party.