@flystep's post had the fhr avatar au grip my brain within moments and rattle it violently.
while i think you could make it work in any avatar era, my own bias goes to korra!era avatar. Mostly so I can shove Chen inside of one of those mechs.
Quick initial thoughts for the RO's + some other relevant characters and what kind of bender/non-bender i think they could be:
Ortega
A firebender, specifically one with the lightning sub-bending ability. Which they use more than the actual firebending.
Chen
Non-bender, earth-nation heritage (family is from Ba-Sing-Se). In an older era he would be a martial expert, but in the korra!era he would operate those mech-machines.
Lady Argent
This one im hesitant on but; Lady Argent as an earthbender and metalbending prodigy.
Herald
Airbender is pretty obvious isn't it? Wouldn't work so well in the 100y war era. Although straight up flight is only possible if one 'loses all earthly attachments', and i think herald has a few too many of those. So regular airbending flight with the help of gliders it will have to be.
Dr Mortum
Non-bender. Wears clothing that does not identify them as part of any nation specifically. Still a scientist, likely an inventor.
Hollow Ground
Waterbender, born in a fire nation colony, mixed nation descend (to make it easier for your sidestep to be any other kind of bender). A bloodbender on the levels of Yakone and Noatak. Does not need a moon, and can fuck with people's bending.
Sentinel
Airbender. Less of the flowy movements and uses airbending more in the way Korra bends air. Powerful gusts of wind. Moving like a boxer, less like a monk.
Anathema
Waterbender. Uses water laced with other substances that they carry, really add a kick to their fights.
(Bonus) River Becker, my own sidestep
I'm inclined to make river a waterbender/bloodbender as well. But that is like giving candy to a baby. Giving the water nerd exactly the power he desires? nah. He could alternatively be an airbender (+spiritbending as sub-bending) or a firebender i think. Or i shall be merciful for once and give him waterbending as he desires.
These are just my interpretations ofc. I'd be interested to hear other's thoughts on character's bending/lack thereof, as well as what they'd imagine their sidesteps would be. 👀👀