Well, I think that it was one of the themes in the original show that bending wasn't really as limited and narrow as most people treated it as being. The masters are really just approaching the subject more flexibly (and from a different perspective) than the average bender has done up until that point. Iroh studied firebending with the Sun Warriors and the dragons, and even looked for insights from waterbending, which we know is where he learned to channel lightning. Toph learned to earthbend from badgermoles, who, we are told, taught the very first earthbenders, Oma and Shu. Hama learned bloodbending (and other seemingly advanced techniques) because she had been pinioned hand and foot in a firebender prison, and had few resources to draw on. (No training at all, there, just dogged persistence.)
None of that requires fantastic amounts of power or skill, just the kind of left-field inspiration you don't get studying katas with Master Yu, and the dedication to pursue it. Like Toph's sudden insight about metal being another kind of earth - it was just an idea that nobody had had yet, hence the firebenders imprisoning earthbender prisoners on a rig made of steel. But after Toph did it, it was something you could learn by studying it with Toph, or perhaps by reproducing her success on your own, if you knew about it.
So really, I think that rather than being of a different order that "standard" bending, lightning was just...you know, a new idea and a relatively tightly guarded secret.
Well, thing is...with Korra being Water Tribe, they haven't really touched the old good guy/bad guy dynamic from before. In spite of certain outliers (Long Feng, Hama), there is little reason to see the Earth Kingdom, Water Tribe, or Air Nomads (such as there were) as potentially antagonistic. Closest we get is Chin the Conqueror, way back in history, eating up the mainland around the invincible Ba Sing Se. And he barely exists in-narrative, he's just some flashbacks.
Seems t' me, it's more of a challenge making a firebender heroic, after spending so much time making them the authoritarian, genocidal conquerors in the first one. Plus, I kinda have this idea in my head about siege engine glaciers creeping across the landscape.