I still think Medusa can be a standalone character. IIRC, explanation on an echo included personality and similar proportions to the other character that determines it. I can't remember what made Isabelle a standalone rather than a Villager Echo, but if we do think of Palutena and Medusa, I suppose their body proportion, not counting the snake hair, would make them somewhat similar, but her personality is completely different... Other things to consider:
-Snake Hair ((though she could always have an alt featuring the long black hair she had in the anime short as an alternate), There is a chance there may be a move that involves it.
-Sharp nail claws instead of a shield like Palu has. Staff or claw swipes can replace the moves regarding Palutena's kicks.
-A more ruthless personality, which could mean more aggressive and relentless attacks as opposed to Palutena's graceful movement.
-Easily different moves rather than using Palutena's "Powers" (yes I know Viridi grants Pit it also), and we've seen the possibilities her boss battles could give us.
Honestly all the echo stuff still confuses me to this day and how they do it sometimes, but I'm certain there's chances she can be standalone, and she certainly has the uniqueness to be that. Plus I would prefer her to be that way so KI doesn't have to suffer with having 4 characters with 2 being echoes. But you know at this point, her just being in, whether Assist Trophy, Boss, or the much preferred playable, would satisfy me knowing Medusa gets the recognition she deserves, as well as being in HD greatness.