Wouldn't it be cool if Eve, Tahm, Shaco and Nocturne became Demons?
Just like in Full Metal Alchemist, they could represent "sins"-- Mind you not necessarily the Deadly Sins, so as to open champ concepts to more ideas. Their "back story" could be that they torment people based on their specific "sin", and have spread around Runeterra, having different territories where they prey on people.
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Tahm: Gluttony (obvious)
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Eve: Lust (obvious)
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Shaco: Sadism
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Nocturne: Wrath (maybe there are cooler themes to explore here)
is a demon, it's confirmed.
can possibly be reworked as demons
And
is a demon as well, most likely, since he's the fire spirit that fought Renekton and Nasus