Yo yo yo I'm back after a brief hiatus from the game. Glad to see stuff's still happening. Now, it's time for another loooooong wall of text about my thoughts (yay!).
Anyways, I was reading through some of your ideas about book 4, and I thought, "what about Zaheer?". Think about it, the fact that Zaheer is a villain makes him nearly perfect for the slave route. I guess his romantic relationship with P'Li and teammate relationship with Ming-Hua will kinda have to be shafted if he is to be used for the slave route, as you can't really mix romance into the slave route. Regardless, he ties into the two of them, which is a bonus. Aside from those 2 girls, the rest can just kinda happen off of his little crusade against Korra. Since it's the slave route, I guess he'd be able to capture Korra and the other girls of the show and be able to do dirty stuff with them and/or command them or something. The only real issue here is that Zaheer never really "learned" airbending per se, he kinda just mastered it after the harmonic convergence. Or, I guess we could just use some random dude for the slave route, but it'd be a lot more boring.
Thinking about book 4 further, it's really such a mess in terms of what character should be used. Korra fits the airbending plot, but is a girl, so unless we implement futa action (hellll nah for me) or she just goes around fucking other guys (also a no for me; I prefer one guy and many girls), she wouldn't really work out. Mako would really be the best in terms of the relationships in the game, but like some have said already, he's not an airbender. One thing that MITY and co. could do is somehow give him the ability to also airbend. LoK did introduce the idea of average citizens having the ability to bend multiple elements. It's also not uncommon for the devs to add in their own story ideas, so who knows, maybe an airbending-Mako would work? Now if have to go for an actual airbender, there are a few to consider. Tenzin wouldn't work, as he's already mastered airbending. He's also a bit old for my taste for a protagonist. Bumi suffers from the same problem as both Tenzin and Zaheer (too old and immediate mastery of airbending), so he's out too. I think Meelo might have to be the one, as we could kinda do some weird time stuff and age him up to be the same age as the Korra gang. If we do have Meelo as the protagonist, we could have some of that sweet, sweet incest with not one, but TWO sisters. Sounds pretty good to me, as that was one of the best parts of book 2. We could also kinda go through his airbending training while progressing with the story.