To expand a bit on my idea, Isaac's moves would be based on his Psynergy of Earth and Plant Matter, and most of his moves would charge in three stages, all attacks can be launched immediately, but charging will make the end result of the attacks larger, more damaging, or faster. For instance:
Up Special: Rockfall, Isaac summons a boulder under his feet that propels him into the air, the boulder itself acts as something of a platform, like a reverse Sonic Spring, instead of it immediately dropping, it carries Isaac into the air, and then will fall under its own weight as a projectile. Isaac can actually jump off the boulder too, if he has any jumps remaining (if he double jumps and then uses it, he cannot leap off the boulder, although it will fall faster than he does). Charges in three stages, Rockfall only has a single boulder, Rockslide rockets Isaac up higher and includes a few additional boulders at the side (not additional projectiles, just makes the core boulder 'bigger'), and Avalanche which launches Isaac up the highest, and has several additional boulders increasing the attack's overall size. As said, despite its appearence being of several boulders, it will always count as a single object.
Down Special: Quake, Isaac slams into the ground and causes a ripple effect that damages and launches opponents. When used in the air, it will quickly rocket Isaac towards the earth, and the damaging aspect will not take effect until he hits a platform. Can be cancelled. 3 stage charge, Quake has a simple ripple, low damage and launch, Earthquake is a larger ripple, whereas Quake Sphere created the largest earth ripple, with a smaller, second ripple right behind it.
Normal attacks would be based on physical moves used by Isaac, including Skull Splitter, Ragnarok, Odyssey, Sabre Dance, and Annihilation, only instead of a giant sword, it would use the general idea with Isaac's own sword instead.
Grabs would be based off Isaac's out of battle moves, such as Move, Grind, Tremor, and Scoop, for instance.