Isaac has so many attacks that can be used in his moveset.
You have the psynergy out-of-battle moves - move, lift, growth, carry, retreat - the djynn and the summons, Isaac's in-battle psynergy moves, like Ragnorak, Mother Gaia, and Quake Sphere.
In Smash, Ness and Lucas use moves from their party members from Earthbound and Mother 3. Isaac could do the same. With this possibility, you also have Garet, Ivan, and Mia's out-of-battle psynergy - force, halt, cloak, reveal, douse, whirlwind, frost - their djynn and summons, their in battle psynergy like Heat Wave, Eruption, and Scorch (IIRC) for Garet, Storm Ray, Plasma, Whirlwind, and even stat boosts like resist (which could work like Cloud's limit charging) for Ivan, and Frost, Tundra, and Glacier for Mia.
Isaac is probably the potential Smash newcomer with the most moveset potential. There's so many ways Sakurai could go with representing him and GS as a game through his moveset.