I certainly agree that Korra/Asami would make a much more interesting group than Korra/Mako/Bolin. However, I don't really see Tahno as a much better replacement. Perhaps an older person to top off the trio. One of Lin's lieutenants, perhaps. I'd say Lin herself, but that's too much estrogen for Team Avatar. It needs a dude.
Could be nice to get out of the city for at least an episode or two.
On another topic, I just rewatched the first season of Last Airbender and, damn, Katara sure does progress with her water-bending lightning quick. In the first episode, we see her struggling to perform a relatively simple move. By the end of the season, after just finding a single water-bending scroll and practicing with Aang, another novice bender, she's able to pull off some pretty advanced shit when fighting Master Pakku. Oh, she also randomly discovered how to use bending to heal. (That said, I really liked her fight with Pakku, especially when she "dug in" and froze her feet to the ground after flipping over him.