Ylem - "primordial substance from which all matter is formed", the original condensation prior to the big bang. It'd be like a character that is a swarm of nanabots, but it'd be more natural, perhaps technological in the sense of the Aurum from Kid Icarus Uprising, like divine mechanics. It'd move, attack, and defend by transforming itself in various elements and constructs all around perhaps a main core and design motif to give it distinctive features to grant it identity.
Another idea is having two separate characters that are connected via story--a demon and human swap bodies (probably initiated by the demon against the human's will as a form of escape?), as in one is a human with a demon's heart, and a demon with a human's heart. Perhaps done many times before, but that could be interesting.
Which reminds me of the concept of nephilim--how the child of an angel and human makes, oddly, a giant human who behaves like a demon. Much different than a lot of conceptions in modern stories about nephilim, but very fascinating from a thematic standpoint I think, and could perhaps translate into a unique fighter.
A fighter based off of King Midas could be quite distinctive as well, where much of his body is made of gold, jewels, coins, etc. and uses his wealth generating power and body to attack in all sorts of ways, all the while looking pretty different visually.
Or maybe an old man/woman character that can transform into their much stronger youthful self via some sort of immortality temporarily. I've no clue how to make sense of it, but someone whose story and fighting revolves around acquiring and prolonging youth would be pretty cool.
Which then makes me think about astral projection for some reason--having a character who can disembody their spirit from their body to perhaps fight or travel. I like that idea too.
In terms of greek mythology, Epimetheus is a character not often remembered--the Luigi to Prometheus haha. But he's the source of all of animal's skills, senses, claws/fangs. He's like their father/mother, a druid almost, the hero to animals whereas his brother was to humans. He also was the one who accepted pandora's box lol, which is another character that could be interesting--a woman with a jar or some sort of container that she unleashes rather unpleasant things from to fight with.
Alright, I'm tapped out at the moment. Hopefully something inspiring occured haha.