I reckon some heroes could fit into multiple elements, while others (like Ra) are very clearly representative of just one. Let me go through the list and see what I think...
Ab'Zero - Fire and Water, for obvious reasons. Okay, the water is frozen, but it's still water.
Bunker - Probably Earth for the strong, tough armour, but Fire for the...well, firepower.
Fanatic - Air because of the wings and ability to use them, but Fire for the holy-righteous-wrath-burn-you-heretics powers :).
Haka - Earth, of course.
Legacy - Probably Earth and Air, because he's physically strong but he can fly. In the alternate timeline, he's probaly fuelled by fire as well (his anger).
Ra - Hmm, I wonder... :P
Tachyon - Possibly Air? She can travel so fast as to be a blur, and if the card names are actually descriptive of the speeds she's travelling then she can actually go at light speed, meaning she'd be invisible. Being hit by an invisible force could be likened to being buffeted by the wind, maybe? Okay, a wind that can punch you in the face, but still...
Tempest - Obviously Water and Air.
Visionary - I'm not really sure where psychic powers could fit. She can change the elements used by other people, though (Twist the Ether), so maybe she has influence over all four, to some extent.
Wraith - Probably Earth, because all her attacks are physically based (seeing how she doesn't have supernatural powers of any kind)...though she does have several thrown weapons so maybe she sort of covers Air as well...
Mr Fixer - Probably either Earth or Air, or some combination of both - he's got the physical element of being a mechanic and knowing how to hit people really hard, but meanwhile he meditates and is internally calm and stuff for all his martial arts styles. I suppose Monkey Fist lets him cover all elements, though.
Expatriette - As another non-powered pwerson, I'd say she fits under Earth for that aspect, but like Bunker her guns would allow her to cover Fire as well. Depending on the Ammo, she can also cover Water. I'm not sure about Air...but bullets fly, right? ;)
Adept - I'd say Air for all the song stuff, so that'd be his primary element, but then he can cover any of the other elements with various songs, eg Counterpoint Bulwark's protection could come under Earth, while Scherzo of Frost and Flame would obviously cover Fire and Water, etc.
Nightmist - Considering the whole mist thing, presumably Air and Water...
Chrono Ranger - Earth for his general physical nature (as he has no powers but gets his abilities from his gear, even if part of said gear is now an interchangeable part of him), maybe Air for the "flying" through time and stuff, and Fire for blasting the crap out of stuff with things like the Masadah.
Omni-10 - Earth for being made of metal, Fire for things like the Plasma Cannon and Rocket Punch (which is sort of Earth powered by Fire), Water for the Innervation Ray (Water generally represents healing), and Air for...erm...well, he seems to be hovering in the air in several of his cards...
Scholar - Earth for his general "Hey, chill, man" attitude, Water for the healing, Fire for blasting the crap out of stuff.
Unity - Earth because she makes her bots out of metal, but the bots themselves possess stuff like Fire (eg Platform Bot) and Water (Cryo Bot).
Something along those lines, anyway ;).