Board Thread:Questions and Answers/@comment-9205010-20140321044627/@comment-6532078-20140321203440

It's not stupid question since all known King Vessel are royalties/rules of their tribes/similar xD I think they can be from the slums though. Let's take Alibaba as an example - Aladdin chose him not because he was a prince but because he believed Alibaba would be a good king. Even better example is Sinbad, but since I don't know if you're reading AoS manga, I'll try to keep the spoilers to minimum.

What is known from Magi, Sin founded Sindria after conquering several Dungeons. He was from family that was not liked by the royals for certain reason, yet because he desired the power to change the world - the king's power - he was able to obtain Djinn. And he was pushed by one Magi to conquer it (and explained what the hell was this all about). On the other hand, there is also a character that came to the Treasury Room before Sin in the another conquer, but the Djinn was seriously disappointed that this character desired the power not for himself, but for his country, so he didn't want to give it to him.

So, I believe that everyone can be chosen by Magi (or Djinn) as long as they qualify as kings. Since royalties can get into conquer with their whole armies and are taught how to "rule" since they were born, they probably just have better chances. Plus Magis seem to like being in the center of their countries (except Yunan, who is just traveler for reasons yet unknown and Aladdin of course, who didn't know anything at the start of the manga).