I was wondering about that too.
All 3 sisters are Naga. Medusa have Monster subtype, while Euryali have Divine instead, probably because all Naga in Mist of Scales consider her a god to worship. Her backstory might be retcon a bit now though. Medusa is almost implied to be be Euryali’s mom, not sister.
Undine/Poseidon reference lead me to research a bit into Greek mythology, and found out about 3 Gorgons; Stheno, Euryale, and Medusa. I didn’t know the connection before. So they’re all accurately represent in the game now. 
Maybe Stheno having a Fey subtype might have something to do with that? She is also the only one of the sister to not have snake hair. Maybe she left her home, learn fey magic in exchange of her Mythic power, and become Bright Forest resident?
Also, spoiler alert for next Campaign story, all 3 sisters were born in cave, like in a mythology, which later become Hellcrag. Medusa is even fought there in the Campaign’s final week. So technically, Medusa should actually be Hellcrag troop instead of Mist of Scales.
I like how Medusa story is deeply connected to Hellcrag history. But for artwork, I agree Medusa is right where she belong. I will give Stheno a pass for above reason in my head canon. 
As for heroic gems, they doesn’t seem to be a new one for next campaign. With heavy focus on turning-to-stone, stone blocks might start falling instead? That would be quite annoying.
Also, some sneaky info were added for the Campaign after already. The name is The Prismatic Key, with heavy focus on crystal and light. So I assume there might be a new heroic gem based on reflect status?
Strength also seems to be the next tarot in line, probably connect to the campaign.