Isaac has a so many elements from Golden Sun that not all of them could be covered in a single move set. Isaac has a sword, items (example: oil drop), psynergy (many kinds to choose from there), Summons and Djinn. The best character I can relate this to, that is in Smash, is actually Cloud. In Final Fantasy, Cloud has several different actions: standard sword attacks, Limit Breaks, Materia, and summons. Yet Cloud's move set in Smash is primarily focused on his sword attacks and Limit Breaks with no summons or Materia used (not even for his final smash).
We could see a similar situation with Isaac where his move set is his sword and psynergy. I know people want him to do more than swing a sword (I feel the same way) but, swinging a sword is likely going to be his jabs, tilts, aerials, and maybe even his smash attacks because it is easier to do that then implement a psynergy attack.
Edit: Even if his move set does not use Djinn and summons, they could still appear in the game in some other way (assist trophy or stage hazard/element).