Returning to the three heads of the dragon theory, I think that Bran is a good candidate as the crow promised him he would fly. Also he could be a knight on a dragon because it wouldn't matter that he was a cripple.
However all the other dragonriders have been Targaryens, so maybe only those with 'dragon' blood can be riders. This rules out Bran and probably Tyrion but means that Jon and Roberts bastards are all still in the running.
However all the other dragonriders have been Targaryens, so maybe only those with 'dragon' blood can be riders. This rules out Bran and probably Tyrion but means that Jon and Roberts bastards are all still in the running.