This has probably already been mentioned by now, but I think it would be pretty neat to have a separate Book 4 and LoK book(s). I'm actually invested in the plot that the writers have developed with the MC (NOT Aang/Zuko, but the dude possessing them), the naga and painted lady, Katara knowing more than she's letting off, and the culmination of the harem from all the previous books. A book 4 with more secks opportunities with all the past grills and possibly some new ones *AHEM* Yue *AHEM* would be really cool in my opinion. I'd hate to see all that good writing go to waste when a huge time jump happens leading up to the Korra stuff.
Also, on the topic of more books than the assumed 4, it'd be nice to split Korra's stuff into multiple as well. As it stands, we'd essentially be taking an entire series and condensing it into one "book" for this game, which could prove to be problematic in terms of plot progression and character development. Think about something like this: a book dedicated to the good guys (Tenzin's family, the Beifongs, Korra, and Asami), one for the bad guys (Ming-Hua, P'Li, and Kuvira), and one for the other lesser-known characters/wrapping up the plot. I know more books might anger people, as it would obviously prolong the time it takes them to finally fuck their dream girl, but it'd certainly be a lot healthier for the plot and characters.