The first plausible break from the Aang x Katara ship, Haru had a lot of Kataang fans fuming. Although Haru is more mature than Aang is, and he and Katara have things in common, would a relationship between the two really work?

I suppose it depends on whom Katara would want to be with. Aang clearly has a crush on her, and Haru clearly respects her and is at least starting to crush on her. Katara is protective of Aang, but she also can relate to Haru. No matter how you spin it, they both have a fair possibility.

However, because Katara chose to continue traveling with Aang instead of helping Haru reclaim his village, the odds of her and Haru hooking up have dramatically went down. Don't you know, traveling buddies always get closer than those do in a long-distance relationship.

But let's say, hypothetically, that Katara ditched Aang and went with Haru. After the rabid AxK typhoon chilled out, I think Katara and Haru would make a cute couple. They have similar interests and personalities, and have faced similar situations, so they could support one another. Although... because of their combined "nice-ness" and overall calmness, the relationship could get a little boring. Although we haven't seen enough of Haru to make a definite decision, from what we've seen I think my hypothesis is right. Haru & Katara would make a nice couple, but Katara & Aang harmonize and balance each other out much better and that would bring excitement to the relationship.

Still, Haru would make a good, supportive friend for Katara. And if he ever returns in a later episode, whose to say what twists might be taken? Who knows, maybe the HxK path might be given a little more ground as we see more interaction between Katara and Haru.
But only time will tell.