If fiddle would become a demon like TK, Eve or Noc, I think he will feed on the people's despair rather than fear because I think Noc would fill that role in his VGU.
Imagine:
Tahm Kench - feeds on desires, tricking people on what they want but in return they would be eaten in the end. The only thing that could prevent him is by declining his offer.
Eve - feeds on pains, seduces people but in return they would be killed and eaten. The only thing that could prevent her is by not touching her.
Nocturne - feeds on peoples fears through dreams/nightmares but in return they would be brain dead(comatose that you can't wake up forever) and he can posses your body. The only thing that could prevent him is by waking up the person he's invading in their dreams and by conquering your fears present in your nightmare/dreams. (seems like the pennywise+freddy krueger of runeterra).
Fiddlesticks - feeds on people's despair. He is the embodiment of suicide. like eve, he cannot touch his victims unless they are in despair mode. Every day, he keeps his victims loose hope by whispering into their ears non stop. The more his victims turn into the dark side, their body starts to deteriorate, like they become very unhealthy/skinny. Not only that, but F in fiddlesticks also stands for Famine, so his victims become more and more unhealthy until they become dead and forgotten The only thing that could stop fiddle is by not allowing you to listen to him and gain hope immediately, which I doubt.
Lastly, speaking of demons, lets throw Shaco into the mix.
Shaco - Feeds on people's malice. He just looks like a puppet but in fact, he is the puppeteer. Once Shaco put his eyes on you, he will inject one of his invisible strings into you body. He will control your body to do malice stuff until you die or he will just kill you if he had enough of you just like a kid that doesn't want to play with his toy anymore. He also likes to have fun with his victims by toying with them. DOING TRICKY STUFF. Not only that, but only those people whose his victims are the only ones that could see him. Making his victims crazy/abnormal because pips wont believe them until they snap out of it and making them insane so that shaco could feed into their malice. The only thing that could prevent him from his spite is by saying/guessing his name out loud, causing his string to vanish. But i doubt it if any one could guest it.
If this is the case for shaco, I could see him be affiliated with jinx. Mini theory, Shaco cause the craziness of jinx.
has a really good theme going for him, Fear. Honestly, I can just see him using someone's greatest fear against them & just feeding off their sheer terror.
, pertaining to the fear of the dark).
