Apparently someone in Nickelodeon Mexico screwed up and put episodes three through six on their servers, and some intrepid fans have obtained copies and subtitled them. I haven't watched these episodes myself, but I have a couple of key plot points that pop up every few seconds the thread pops up that catches my eye. No news when the episodes will be released, but I can't imagine it'll be too long from now. I've heard rumors of September, but yeah. Rumors.
Remember, this is just stuff I've skimmed over on threads dedicated to this topic, so not particularly reliable. There's a Combustion Girl, who is the girlfriend of an evil Air Bender. An armless Water Bender. Toph has another child from another father than Lin, who has a daughter named Opal. Mako's and Bolin's family. Ba Sing Se government still evil.
Over all the general consensuses is Book 3 is a pretty good season, but to be fair the biggest problem with Korra has been pacing and the end of season fiascos.