For me i don't really think controlling somebody who can't learn air bending is such a big deal or even freak everyone in the end we are the avatar or to say a future avatar.Even as you said in book 2 Fire we get to create a character and that character has 0% in the story for me that would freak me more and not if somebody we control can or can't learn the element.As i said before is us who we control not him if he can't learn it we can it is just that we can't go ourselves so we have to control somebody. Another reason is that there are a lot of people who first see who they control before playing a game and some don't like playing as Old person,Female protagonist,Futa,Shota-Loli,Trap,Gay,and many other things.As for me i would rather play somebody young and not play as Bumi i mean the dude is like 55 to 65 years old he can be their father that for me would be little weird.
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2. This is just my thinking i don't think this will happened low chances.Maybe because it is the future we can have a chance in this book to play as us and not get to control anyone.The others are old Aang is dead Zuko is old.Just saying but we will see what will book 4 bring when it comes out.
3.Why i think Mako is the best to control in book 4 is that he was in a relationship with the two main girls Korra and Asami but now they are former lovers.So that can make him the best for this role in lover and slave route.Lover route changing the cartoon story and having the two of them as his lovers and Slave route him mind controlling them or by other means making the his slaves.
In book two, he wasn't just anyone, he was a random 'Fire Bender', that is the key, there was an excuse for him to be learning to bend fire, which, if you have forgotten, is the entire point of all of this.
In Korra's day, until she did the thing and brought air benders back into the world, the only ones there, were Korra, and Tenzin's family, the youngest of whom was already a better bender than her. If anyone else were to be trying to learn air bending, that would freak out anyone else in the game world. It has to be possible for them to be able to learn air bending, because there is no reason for him to be there if he can't. The sex is our reason for going but his is to learn to air bend, without that there is no plot.
I would rather not play an old guy either, but if I were going to, Bumi is the one, he is a main character, he is funny, and he is a noob air bender. He meets all of the criteria. I don't really care who we posses, because it is self insert. A lot of people don't like a lot of things, that doesn't stop them from being in the game. If you remember, in Book 2, in one route we played a random fire bender, and in the other we played Zukko, so the same deal could happen again here.
Makko (his brother, or just about an other main character), was not an air bender though, so he is not possible, the character
has to be able to learn it, or the game has no point. That is the entire point of him going there, it is critical to the plot.
To me, the most likely is some random person who got air bending powers when so many others did, at least for one of the two routes. Assuming it is a male MC, as usual. If it is a female MC, then it pretty much has to be Opal.
As for playing as us, we are not yet born in that age. We are at least 2 avatars after Korra, minimum. A new avatar cannot be born until the last one dies. So at minimum, assuming Korra died at the end of her story, and the next avatar died at 18 when she was first old enough to do the sexy ritual the Painted Lady was telling us about, and we are 18, it can't be less than 36 years after the end of Korra's life.