I can't vote because I'm torn between the first two but I don't like time travel/split settings. On one hand, for the sake of NaNo being a fun exercise first and foremost, it makes sense for you to write a style you are comfortable and confident with, and won't get hung up on, for a better chance of success. It should be a stressful thirty days. On the other hand, I am a huge sucker for modern retelling of fairytales, stories with strong social messages, and I think if you simply created an alternate 'verse in which monarchy/medieval castes were never done away with despite the advancement of science/industry/etc. it could still work without too much difficulty. I think I'm going to be excited about and enjoy the outcome regardless.