Asgardian spoilers
So why did Billy change his code name from Asgardian to Wiccan? To avoid the jokes. Beginning in the first issue, hints were dropped about the nature of Billy and Teddy's relationship, and while there hasn't been an official "coming out" yet, it's pretty clear that the two guys are an item. Therefore, no more Asgardian.

Actually, that's just half the reason. Though he is, at a glance, the younger version of Thor, Asgardian's powers include more than lightning and flight - he can cast spells using a technique he picked up in self-help books, chanting a simple mantra and picturing what he wants to change. Since he's got the magic thing going on, Wiccan makes sense. But what's up with that power?

Billy got beaten up at school quite a bit, and would hang around outside the gates of Avengers mansion to cheer himself up. One day, he happened to meet his favorite Avenger, Scarlet Witch. Wanda told the boy he should stand up for himself the next time it happened, and touched his forehead, healing his wounds in a flash of red light. A few weeks later when he saw another kid getting bullied and tried to help, his powers manifested. So did Wanda grant Billy powers, or what? Well, maybe, but not in the way it seems.

Using Vision's emergency program, the team found another teen with the potential to be a hero. His name was Tommy Shepherd, and he looked almost exactly like Billy - except for his platinum blonde hair. So now we've got a dark-haired magician and a white-haired speedster; does that sound like any other sibling pair in the Marvel Universe? Years ago, Vision and the Scarlet Witch were married and conceived a pair of twins, who were eventually said to be purely magical creations of Wanda's subconscious, animated by fragments of Mephisto's soul. Yeah, that makes sense. Anyway, the twins' names were Thomas and William; now we've got the nearly identical Tommy and Billy, with powers that suggest Quicksilver and the Scarlet Witch. Maybe the twins weren't as imaginary as everyone thought.