Obviously not the GM here (and please don't bother to bitch slap me if I'm out of line, DR), but I think it would be much easier and advantageous for characters to be at least nobility, or at least something above a commoner or the like.
From a story standpoint, it's the largest Tourney in quite some time, with lords and knights and ladies from all over the Realm coming to participate and witness it. Even the King is going to be there (or not... Can never tell with that crazy son of a bitch). With most people taking lord and ladies characters, bastards and legitimate, I think that it makes more sense to do a character that would be more likely to be around them, than say a commoner. Can't have the peasants mingling with the King and Prince and their posse.
Wildlings... Dothraki... Commoners... From my own perspective, it seems like it could be so difficult to get them involved and interacting with characters that are most likely (and realistically) be separated from them. Unless you're doing like a thrall to an Iron Islander, or a paramour to a Lord (or Lady), or a servant, I think it might be a bit difficult to get involved.
Just my two cents...