So, I guess we've got Korra with us until 2015? That's assuming one book per year. If that's the case, that's a good run.
Asami's concept art looks good.
Asami's Corporate Outfit
Asami's Pilot Outfit
And Mako's outfit looks like a Starfleet uniform variant at first glance.
Mako's Starfleet Uniform
Korra's outfits don't look all that different from what she already has. I liked seeing her in the more modern civilian clothing from Book One and I wouldn't mind it if she adopted that style at some point.Tons of Book 2 concept art from Comic-Con, including new costumes, characters, and settings (oh, and some discussion of content too):
http://io9.com/5926144/beautiful-concept-art-illuminates-the-upcoming-book-2-of-legend-of-korra
Asami's concept art looks good.

Asami's Corporate Outfit
Asami's Pilot Outfit
And Mako's outfit looks like a Starfleet uniform variant at first glance.
Mako's Starfleet Uniform