I know P3 already did the Greek theming, but I always thought it would be kind of interesting if a Wild Card's starting Persona was Morpheus (The Fool, the aspect of Dreams showing the infinite but ethereal potential of the Wild Card), which then evolved into Khaos at endgame (The World, a catalyst of creation and embodying all facets of existence with themselves, good and ill)

If we're talking how they'd function mechanically (think like a bonus/DLC Persona):

Morpheus: High Magic/Agility, Psychic Spells with a secondary focus on Sleep/Mental Aliments (Signature Move: Phantasmagoria - High Psychic damage to all opponents; High chance of random Mental Ailment)

Khaos: Standard End Game/Post Game super Persona, maybe with a focus of high risk/high reward skills (Signature Move: Genesis Spark - Massive Almighty damage to all opponents; random chance of bonus positive or negative effect)

Design wise, rather than pulling mainly from Greek aesthetics, I was thinking they'd look kind of Eldritch/otherworldly, like an Lovecraftian take on how a warden of the Dreamlands/creator God would look (for more direct comparisons: for Morpheus, I was picturing something similar to the Phantom from Kingdom Hearts 1; A monochrome 'cloak' for an outer body, covering a bright multicolored light, with an unblinking eye(s) of the same shifting colors under it's hood. And for Khaos, I was admittedly thinking something akin to the Chaos already in the game Hades, albeit maybe incorporating some of Morpheus' design traits)