Demons are pretty cool so I could see a demon champ coming to LoL but if they did, it wouldn't be a stock demon. We'd find our own take on demon. There will never be a heaven and hell in LoL that is just too cliche.
Similar to Kayle though there is no reason why we couldn't have a demonic champion with a unique LoL take.
Because Kayle and Morgana are not totally cliché ? Then why is Morgana portrayed as completely evil in every part of the lore except her own ? (and even then, her voice lines are much more evil thatn I'd expect from someone who is trying to save her people...)
We saw that once more with the "Aether Wing Kayle" skin where she's again portrayed as completely angelic while Morgana is shown as "evil".