Oh I've been waiting for this one.
See, Agnes and the rest of the BD cast can class change into any class in the game.
How I'd translate this into combat, is to make them the ultimate Frankenstein character, using componenets of every Smash character thus far.
Every move would change them into a different class costume before switching back to their normal costume.
Imagine if her forward air was Sheik's or her bthrow was Zelda's or her FSmash was Cloud's. So on and so forth.
As for the iconic BD system? Possibly a reward for perfect shielding aka defaulting, allowing the player to build up brave points to power up their smashes with additional hits. Of course, you can go into "debt" with this system, meaning you'd be unable to use those powered up smashes until you defaulted enough to get out of the red.
And their Final Smash could be one of two things.
They'd activate Bravely Second to stop time and do their potential attacks:
1. Summoning
They're probably gonna go with Susano-o on this
2. Music Attack
As a bonus effect, her theme would play even after the final smash is done, giving her an aura and a buff to her abilities.