One of the worst sins I commit as a gm is over planning. One of my players once said that he felt like I always had a plan for what was over the next hill no matter which way they went as a party.
I blushed a little then had a group of gnolls with polearms attack the party. In truth a lot of planning went into my last fantasy world. I mapped things out in minute detail. I made up a tree for language evolution and relationships. I came up with several major pantheons and worked out what the gods were doing when their people went to war with each other.* With all that stuff worked out it was very easy to fill in the cracks fast.
Happily I don't have that free time anymore and sadly don't have the energy. So while I like to have an overview I'm not figuring out every store on Merchants' Row or what orcs have for supper (smaller orcs.) I'm also realizing more and more what I stat up and give my attention to should be things that involve the players. It makes it easier for them to sit through the inevitable hosings I'm so infamous for.
Anyway my current crews likes are my first priority, in particular my newbie (I know she loves Minecraft and robotics, a dungeon that is an abandoned mine filled with clockwork automata seems in order.) Her parents are harder. I know they're huge fans of Sleepy Hollow and Once Upon a Time. Having a headless yet charming prince doesn't work. I'll think of something eventually. The first session is going to revolve around the reveal that this world is their old campaign world that underwent a horrific change. That sort of fits in with OUaT. I can add the conspiracy to keep things getting worse for a Sleepy Hollow vibe (though this is sort of like the End Days happened already and the good guys lost.)
I also have to remember that one of my players is in middle school and doesn't need to know her daddy played a boozing, wenching sociopathic Hobbit cardshark and her momma played an elven grifter with sticky fingers and a libertarian view of law to say the least. They can break that to her when she's 18.
*It turned out the gods natural form was a bunch of cute li'l tweens who were playing an rpg while hell was breaking loose. My players ran into them while on a hop between dimensions. They were surprised but not shocked.