Alight, so far I've gathered a few of everyone's comments and questions and I think I can summarize them in one post. If I miss any, feel free to tell me xD
About the fighting spirits thing, we could if we were to stick with how the show doesn't have any deaths really, (Besides old age.) If you've noticed. :/ According to the show, you can imprison them but it would require all four elements and since there will be no avatars, four people of different elements would have to help. Otherwise, we'd just have to drive them back temporarily each time or have something that can send them to the spirit world. (Perhaps if its set a little while after the current episode, Future Industries has made some technology or something, but that's a long stretch. Otherwise, I'd recommend human enemies that can be captured more easily and put into jails and such.
Another thing, about Metal Bending. I think I'd like to stick with the story on this one. If you want to be a metal bender, your character should be in the police force, right?
If anyone would like me to post a brief summary (In which may or may not have spoilers) about how it transitioned from Aang's time to Korra's time, let me know here and I'll post one here in a spoiler bracket (If someone tells me how since I'd never done it before. xD)
Also, air bending would be a stretch too, and we'd have to play far enough into the future that Tenzin's kids had kids or that Tenzin's kid's kids had kids. Otherwise, we could go just a tad into the future and any air benders would have to play one of Tenzin's kids which, I didn't really want to do since I like original characters xD
I'd probably be a water bender since shortly after watching all of the legend of Korra in two nights, I had a dream I was one and was totally surfing on water with ice. So I think that fits me the best xD