With the announcement of a demon-themed underdark adventure as the next AP, and my own love of Warlock Flavor combined with a dislike of Warlock Mechanics, I thought I would put some ideas for Demonic-Reflavoring of current melee classes.
Elemental Monk Mechanics, but flavored as partial demonic transformations. instead fo simple unarmed strikes doing more damage, you grow demonic claws. Instead of unarmed attack bonus actions because of training in Martial arts, you get unarmed attack bonus actions because you grow a demonic spiked tail. Breathing fire and roaring can be part of the elemental things...
Totem Barbarian mechanics, except instead of an Animal totem, your "totem" is a demon. Maybe you made a pact and consumed demonic blood that causes your rages... Like the orcs of Draenor. I suppose that could work for the other Barbarian path, too, I just don't like the other barbarian path.
Eldrich Knight - Familiar could be a demon in disguise... it could be the teacher.
I think those are the easiest classes to make demonic. Can anyone think of others?
SterlingRat wrote:
HermanTheWize wrote:
a Vengence Paladin ... much like the classic TV show version of The Incredible Hulk, "Don't make me angry. You won't like me when I'm angry."
Not only does he single you out for homicide with malice aforethought, when he kills you your immortal soul gets suborned and horrible things happen to it, too. 
Cleric can easily be a Cleric of a Demon Lord without any real modification... several domains would work (everything but Life and, maybe, Nature)
Oathbreaker Paladin can be given an 'Oath of Evil' of some kind for a Dark Knight vibe
Sorcerer obviously can be Demon oriented; either as a Wild Sorcerer just as is or a very slight reflavoring of Red-Dragon Sorcerer
And don't forget the demon aligned possabilities of characters without an explicit Demonic Power Source... Knowledge Bards/Summoner Wizards seeking forbidden lore... Deadly Assassins eliminating enemies of a Demon Cult... it takes a lot of folks to really damn a world after all