Is there a way to do any one of these (on PC version):
Start a new game with an old character keeping perks and level
Reset the main quest line back to the escape from Helgen
Reset entire world except character
Answer to No. 1 - No, I don't think it's possible, I've tried many times but never got it to actually work...sorry
Answer to No. 2 - You're going to have to start a new game to do that... there's no other way.
Answer to No. 3 - No.