Based on Tahm Kench, Evelynn and this new post from Interlocutioner it seem's as though Demons in Runeterra are personification entities. Beings that embody the qualities of an intangible thing. In this case it seems to be human vices. So what other human vices would be fun to see as characters in the League of Legends universe? Personally I'd go for a Gambling Demon.
I was under the impression that Demons weren't really based on vices, but were spirits that had evolved to feed on negative energies. Fear isn't really a vice, but it seems to be what Nocturne thrives on.
Demons copy emotions Aspects embody concepts Yordles adopt cultures
It all seems to be connected
It will never become a champion (not that I think it should either) but without a doubt there would be a demon exploiting human bigotry/prejudice.
In light of very recent events here, fuck human vices and the misery we bring onto others.
Actually 
is based around all sorts of vices, he just chooses to describe them all as gluttony.
In fact Tahm is already heavily associated with gambling, as there are 2 stories where he interacts with gamblers(in the first one, he helps the gambler to escape his debt only to mess up his life once the gambler found happiness. In the other story he gives a poor gambler a coin to encourage him to gamble even more).
