I mostly want him to represent his Venus Psynergy. Ragnarok for Neutral B, or F Smash (which can also be his leaping sword attack) Move for Side B, but it's a command grab that makes you able to either push people away, or grab and toss with Pound or throw upwards with Lift. Down B could be anything, Spire, Gaia, Earthquake, Nettle, doesn't matter too much. Up B I think Hover or maybe a Vine Ladder could work ? Retreat?
I'm not sure how they could implement Djinn into his moveset. I think a Djinni combined attack , ending with a Judgement Summon would be ideal. Not sure where else it could fit without Isaac becoming too much of a Djinn gimmick driven character... Then again, this is what Sakurai has been doing with the DLC characters in Ultimate so far.
Psynergy could also be represented in Smash attacks. U Smash could be Spire, F Smash the sword leap or Ragnarok, and D Smash Earthquake, which could kind of work like DK's Down B.
I don't expect Isaac to be a fast character, or very good at jumping, probably also a fast faller with low traction. Not sure how heavy he should be, but am guessing around Link's weight?