In v0.0.9, you'll be able to compete challenges and boss rematches for silver coins, which can then be used to purchase special items at the challenge shop! Completing the rematches flawlessly will count toward achieving goals, but won't alter the game's story.
Right now, the rematches are against the bosses exactly as they appear in the story, but I also plan to add a hard mode version to each boss sometime in the future. I want these hard mode rematches to be more than just boosted stats, so I plan to change each boss's move list and their tidal attacks to make a brutal challenge where one wrong move can be fatal. Since these fights are optional, I think making them prohibitively difficult won't be a huge problem.
As for the other challenges, right now I'm just adding defense challenges, but I plan to give each magic school a challenge too. These will involve special enemies that can only take damage when certain conditions are met, and they'll have round limits, so you'll have to efficiently manage auras and MP to deal as much damage as possible to earn a good score.
All of this will be optional, and I intend to make the highest tier rewards combat gear, so if you don't like the game's combat, it shouldn't be too big of a deal if you just ignore it. The rewards will mostly only appeal to players who like the combat and want to do even harder challenges.