The Demons Need Buffs. Badly.
Buffs is plural for a reason.
Right now, if you look at the Demons, 5 of the 7 Demons are Tier 5 out of all 5 Tiers. (Source: https://blitz.gg/tft/tierlist ). They're insanely bad. I think Aatrox is the worst out of all of them; quite possibly the worst Champion in the entire TFT game.
Aatrox outright needs buffs. The others who aren't Brand and Morgana need buffs. And if you go for a full Demon comp you only have one frontliner in the form of Aatrox, so I suggest actually adding in another tanky Demon into the game, Tahm Kench.
I'd like to experiment with the Demons but right now it just feels pointless knowing that literally every other Origin and Class combination is better.
I know the Demon's ability is super strong and that's why they're probably hesitant to do it, but at this point the benefit of their Origin isn't enough to outweigh just how weak everything is. Plus just having a Kassadin with item stacks basically does what the Demons do but with better efficiency because he's not blatantly bad.
demon/blademaster
tier 3
demon/elementalist
tier 2
demon/shapeshifter
tier 3
demon/assassin
tier 3
demon/sorcerer
tier 5
demon/imperialist/shapeshifter
tier 3
demon/archer