Arnook - Water.
Bato - Water.
Hakoda - Earth, for determination and resilience.
Jet - Fire, for his passion, and because I like irony.
June - I don't really know; she shows different styles. Earth maybe.
Kanna - Water.
Kya - Water.
Longshot - Air.
Mai - either Water or Earth.
Mechanist - Earth, because he works with metal and other types of earth.
Pathik - Energybender, which I'm pretty sure he is already, even if just weakly. He's spiritual, and I can't think of anyone else to give this to.
Piandao - Fire, for the Fire Nation, and his forge. Broadsword swinging would lend itself well to fire arcs as well from the sword.
Pipsqueak - Earth, because he's big and strong.
Smellerbee - Air, because of her agility.
Sokka - Earth, because of his sword, and generally being down to earth.
Suki - given her flowing combat style, and her home, even though she's got auburn hair, and is a Kyoshi Warrior, Water. And partially because of AtoMaki's headcanon.
Teo - either Air, or Earth, if he learned metalbending to better interact with his father's creations.
Ty Lee - Air, because acrobat and cheerful.
Ursa - if she's not already, Fire, for passion, and her warmth. That is, the show version, not comics Ursa. -_-
Yue - Water, because Moon.
LOK - I've already technically done it.
For most of them anyways - those I have bolded have been announced.Asami Sato
- Fire, for passion, and her heritage (Fire Nation descent).Baatar Jr
- Earth, so he can learn metalbending, and get closer to the equipment he already uses in show. And to Kuvira...
Hiroshi Sato - Fire, like Asami, but for hatred(wanting to avenge Yasuko's death).
Hou-Ting - I'm not sure on this one. Earth
Queen, obviously, but given how she tries to control everything, maybe a psychic bloodbender who's completely inept.
Izumi - I always imagined her as a bender. What's a Fire Lord without "Fire"?
Lieutenant - Air, for the agility he already shows.
Raiko - Water. Politicians, much like businessmen, need to be smooth, and we've already had two dbag waterbender politicians, Tarrlok and Unalaq.
Senna - I'm pretty sure she can waterbend.Varrick
- Water. Quick, slick, slippery, and business involves turning things back around, like waterbending turns opponent's attacks back around.Zhu Li
- Earth. Determined, resolute, and physically strong for her size given the way she hefts Varrick around.Forgot one; Wu
- I had him getting Earth, to signify a change in his personality, but based on show, maybe Air. He seems to be more about having fun, and he's not really got a backbone, to put it charitably.
And another you forgot, a big one, Pema
- Air. Because she's got four Airbender kids, and was an Air Acolyte. I really wish she'd have gotten bending in the show. : \