Legend of Korra - The Spirit of Competition

Amon was pretty scary... until we learned his backstory. I really wish they had gone the whole "given power by the Spirits" thing, (and it would have been a better way of introducing Vaatu if HE had specifically been the one to give Amon his powers) or something more mysterious and sinister than just Bloodbending which, while it was pretty freaking scary during its introduction, lost a lot of its fear factor the moment Katara proved that you could resist it if you were a powerful waterbender. 
Amon did have potential, and the thing that made him so scary was the fact that we knew so little about him. He was an unknown, a mystery, a faceless menace lurking in the shadows, and had the writers been able to develop him more, I feel like you would be right. Sadly, due to the way Nickelodeon did the series, they had to keep him a one-act villain. Honestly, I wish Amon had remained mysterious and stuck around as the show's main villain (next to Vaatu). 
Ironically, considering your stance on Zaheer, the Red Lotus has the potential to become this in the future (I was actually disappointed that Season 4's story didn't involve them, don't get me wrong, Kuvira was cool and all, but she just wasn't very SCARY). We know very little about them, they're dangerous, mysterious, and we've only been introduced to a few of their members. I doubt that's ALL the Red Lotus has. The exact nature of their leadership is in the dark, and if the writers ever get a chance to make a new Avatar series, then I hope they're given the chance to do whoever the villains are justice.