Isaac

[Bane84]

To quote from the Golden Sun Wikia:

An Adept is a person in the Golden Sun series who can use Psynergy. There are four types of Adepts, one for each of the elements: Venus (earth), Mars (fire), Jupiter (wind), and Mercury (water).

Generally an Adept can only cast Psynergy of his or her element. However, the elemental spirits known as Djinn (singular: Djinni) can affect an Adept's Psynergy, either granting them Psynergies of other elements (if the Djinni's element is different than the Adept's) or making their existing Psynergy stronger.

I would agree that you might want to stick to the Earth based Psynergy. Also, the idea of the different types of Djinn as items is not such a bad idea. When you think about it, we already have attacks with elemental types in Brawl:

Venus -> Earth -> Charizard's Rock Smash, Donkey Kong's Hand Slap
Mars -> Fire -> Bowser/Charizard's Fire Breath, Mario/Luigi's Fireball
Jupiter -> Wind -> Link's Gale Boomerang?
Mercury -> Water -> Mario's FLUDD, Squirtle's Water Gun

Instead of throwing the Djinn, they could probably act like the Fire Flower where you can carry them around and use the A button to attack with that specific element. You could probably use the single Djinn summon from Golden Sun to get ideas on how they should strike.