Isaac has three games to his name and a dozen years of history. The latest Golden Sun game had a post-Brawl release and was shown off as a significant upcoming title during two Nintendo E3 press conferences in a row. Isaac's got seniority and a previous appearance in the SSB series as an assist trophy. Golden Sun also had a music track in Brawl as well.
He has incredible moveset potential. As Sakurai noted on the Dojo, Isaac's got a sword but he doesn't need it. His primary focus is earth magic. He can launch stones, lift boulders into the sky, create earthen spires, and cause earthquakes. He has control of plantlife as well, attacking with vines and briar patches.
Secondly, he has his telekinesis. As seen in Brawl, he can manipulate the world with his mind. He can push away foes, grab them, lift them and pound them with his telekinetic powers.
If that's not enough, he has even more moves to draw from. He can halt time, hover in the air, turn invisible, teleport, melt into sand, and create cyclones. He can summon gods to help in his fight. He's primarily a user of earth magic, but in the games he's completely capable of manipulating water, ice, wind, electricity and fire as well. If all else fails, he can whack things with his sword, and and even his swords have special, magical powers.
He can even summon emus.