I personally would want him to use melee with Psynergy as a supplement.
Like maybe down smash could be variations of Growth/Nettle, increasing in size depending how long it's charged.
Then Forward smash could be Isaac stabbing the ground in front of him, with the Ragnarok sword materializing as he charges, then coming down when the attack is charged for like 75% or more, increasing his range.
What I wouldn't like would be super projectile-heavy or zoning playstyle. But who knows, having Isaac play like that might get me to like that style.
I was thinking of having Djinn be more simplified for Smash; like we could have the 4 elemental Djinn fly around Isaac (kinda like the ghosts of Pikmin when they die, maybe more transparent so as to not be very noticeable or distracting).
Isaac's attacks deal really good damage while he has Djinn active and taking damage (like say 30%) put a random Djinn on standby. Each Djinn could have it's own individual health.
So basically as Isaac takes damage, he loses Djinn, and his elemental attacks lose range, or they could just lower his Smash stats.
Make Djinn come back after like 7 seconds of not taking damage. Keep Isaac on the ropes to keep him at his weakest
I feel if they made Djinn an active part of his moveset, he'd be like Shulk. Like activating Jupiter would grant you increased jump and stronger damage on moves that use wind attributes.