I voted for Korea.
Now the reasons why I didn't vote for the others are:
Kazakhs: I don't see a purpose for the Kazakhs. During the 1429-1792 era they were just a huge empire comprised of various 'Hordes'. I'm happy with almost all of the other 'siberian' tribes. And besides, I don't see the Kazakhs making that big of a difference. I mean, the various siberian tribes don't make that much of a difference. They quarrel with each other. Russia swallows them. End. A united Kazakh that encompasses almost all of Siberia ruins the whole, different tribes to quarrel and fight with on your own. Then out of the strongest, to fight against Russia. It's like choosing Milan or something and trying to take over Italy, then the world. I don't need an already united Italy! Ruins the phases of uniting your nation.
Dulkadir: I don't see it being that interesting. All I can see is them fighting the Mamalukes. Then the Ottoman Empire (Turkey) annexing them right away. I mean, Dulkadir would be kind of dumb. Just having Turkey annex them. Now we're back where we started! If they are incorperated, hopefully they won't get annexed right away. Like in 1522 (that's the year they really became part of the ottoman empire).
Taungu: I guess they would be kind of interesting. Something to rival the Dai Viet. But I don't see southeast asia having that much of an impact on things.. Besides, the real power of Asia was in the hands of the 'far east'.
Cambodia: Yet another southeast asian country. I don't see a big deal on it.
Zanj: I guess they could add a little more spice to the african continent. Hopefully they won't get tossed around (as i think) like a little doll.
Forget that! I MUST admit Zanj would be an 'excellent' choice. But only if it was controlled by the computer. Beacause you could watch them rebel against Iraq and then being crushed. But it would be cool watching the rebellion. Then it being crushed. Or would it?... Because playing as Zanj wouldn't be that much fun. Being vassalised (maybe even being annexed) by the middle East. And having Persia, Iraq and most of the middle eastern countries tossing you around. Damn!
The reason I chose Korea was for two reasons.
First off, Korea WOULDN'T be THAT big of a deal either.. Like ALL the other aforementioned countries.
But I think they could spice things up in east asia. Now unlike all other 'other' countries mentioned, I think they COULD be interesting computer controlled and/or human controlled. Now their only impact on the game would be in Asia. Not in Europe. But that's the same rule as in like, the Aztecs wouldn't make that big a differnce in Europe (like the aztecs will invade europe? hahaha). Or England having that big of a effect on hawaii. etc.. I think they would spice things up for the human player/computer player because Korea could rebel against China right away. Being vassalised and all. Making war really interesting early on. And during the time of the 'new' choson, even though Korea was vassalised by China, rebellion because of unfair Manchurian rule wasn't uncommon. In fact, the rebellion I'm talking about in the game would lead to Korea to even invent our own language (Hangul), through prosperity.. And Korea made friends with either Japan/China and made the other the enemy. So they could add favor/unfavor in east asia. Making the scales tip up and down. Getting things interesting. 1592 would also KICK ASS! Cause you could change the A.I. in Japan during that time (Hideyoshi's Rule) to have them try to invade China. You know, like in real life during that time, add tension in East Asia between China and Japan. Drop diplomacy and stuff between Japan and China. Because then you could have Hideyoshi invade Korea, and as the korean country it would be your job to STOP the huge japanese naval and infantry advance!! You could add Admiral Yi Sun-Shin as a leader of Korea. Giving him a huge NAVAL boost! Because he's the one who stopped the japanese naval invasion. And besides military. Korea could also be HUGE religious wise TOO in East Asia!! Because of all the European missionaries going to Asia, many failed to convert in China, so they tried in Korea. And in fact, were more successful in Korea. In fact, I think that's how japan became aware of Christianity. Through being introduced to the religion when they captured Koreans during Hideyoshi's attempt to invade Korea. So Korea could be christian and not muslim. And even have Protestant REFORMS! After Hendrik Hamel wrote his experiences about Korea (the first book on Korea published in Europe), Korea had an evergrowing contact with the western world. So during the first religion reform in the game, when you can convert to Protestantism, Korea should reform just like England. So they wouldn't be just 'another' catholic country. They could be a reformed protestant country too!
Second, I'm Korean. So I would love to see my country in the game. And we have a rich history. So it would definitly be interesting to see Korea enter the game.
But this is all my opinion.