I think the coolest option would be to integrate them into Isaac's moveset. Maybe have a few Djinn which you can switch around (like Shulk). His attacks can vary depending on the Djinn he has equipped, representing different types of Psynergy.
Other than that, I think having them as ATs to attack your oponents would be pretty cool. I'd be good either way, I just want their cuteness in Smash. Embarrass
I have a theory swirling around in my head right now.
So Villager has his pocket icon on his percentage window when he has something in his pocket and Cloud has his limit bar in the exact same location.
Little Mac has his KO meter above the percentage window and Robin has his weapon durability meter in the same spot.
We have Shulk's new way to switch into Monado arts quickly, but he is the only character that has something like this unlike the characters I mentioned above.
I think three options are possible
1. Isaac gets a way to switch his djinn around like that if that is in his moveset.
2. Same as option, but Felix comes in as an echo as uses it so more characters utilize that quick swap.
3. Malos comes in as an echo of Shulk and uses it.
I wish all 3 of these options were true, but that's for Sakurai to decide.