The Makeover Fairy is Jareth's long lost (half-)sister from his father's side!They both Instant Cosplay Surprise for no apparent reasons, appear when you least expect them, have some questionable quirks of taste (pink vs. glitter) the list goes on and on. From his father side: If der Erlkönig decides that you need a makeover he won't stop with just you. THE WHOLE WORLD GETS IT! (As much as we can see from the published parts of the 2012 Mind Screw arc anyway.)
The magic family tree is the unholy bastard child of an elaborate Celtic Knot and the Timey-Wimey Ball.Just look at it! And why not? This is a place where personifications of dreams and concepts can walk amongst the muggles.
After the Dreams arc, Jareth will be skittish around Sarah......both because of her being creepily impersonated by Odile and because he wonders if it's his obsessiveness with her that prevents him from the Character Development needed to stop being a villain (or at least be so bad at being good). He'll get over it eventually, probably with some furthering of his character, and Sarah may allow him a date (or at least have missed him).
Dark!Jareth will be easily defeated in the Dreams arcThis isn't so much the post of one person, it's been noted by everyone on deviantART's forums. Jareth was not cast as the villain in the story of his dream, so his Superpowered Evil Side can't use his 'go with the story at all costs' Freudian Excuse.
The zombie invasion ended because Dresden reverted to his Changes characterization offscreenI can't be the only one annoyed that all the zombies disappeared with no explanation. So I'm going to say that either Harry Dresden or the Doctor figured out a way to make them all re-dead. After all, those two do have something of a track record when it comes to genocide...
The Building is Jareth's new castle and the 'verse his domain.Word of Ashe that "...every magical "court" has a theme... The Goblins are "unwanted".". Now this place is full with story rejects, (fallen sympathetic villains, under-appreciated "heroes", supporting characters sacrificed for the sake of someone else's story etc.) who fit the description quite nicely. Jareth doesn't own the place literally but it's his metaphysical / mystical domain, even if the residents are clueless about it... it's okay he was never big on ruling things anyway. This also explains why is he so territorial about the place and it's residents.
And the child born of shadows is...This page and comes with two hints "the one thing every character ever written, played, sung about, or dreamed about has to fear" and "??? belongs to ???"
(This entry is explicitly added to show what goes on in the comment section! Ant these aren't even all the guesses.)
There will be an omake where Jareth is outraged by Javert's new actorWe all know he hates it when Eric gets new actors, why not Javert?
Dresden will encounter Jareth's dad sometime, and it will be amusingIn the fourth Dresden book, Harry encounters the Erlking and summons him himself. The Erlkonig promises the next time they meet, it won't be pretty. Amusingly enough, the Council sees the Erlkonig as the leader of goblins...
The Erlkönig's reappearance is for some reason he needs a heir badly.He already called Jareth his (possible) heir in the Kid!Jareth arc despite having a daughter, so he probably believes in the Heir Club for Men rule. He already tried all in his power to extort favors from his son (Such Stuff..., the Erlkönig arc could be a failed attempt for this too), pay a hell lot to ensure that he can introduce the guy to his friends (The Wild Hunt arc), get in position that could be interpreted as him doing a favor to demand payment later (the beginning and end of the Dark!Jareth arc for example).
Jareth is the human boy from The ErlkingThe Erlkönig only has daughter(s) canonically also even Jareth threatened Sarah with turning Toby into a goblin... what if that's a power that runs in the family?
Loki and Jareth are half brothers via JadisThis comes from two points 1. All magical people/beings are apparently related so they must be connected somehow. 2. Jadis is the White Witch and queen of winter so all of Jotunheim would fall under her control. Perhaps she realized that she had lost control of her first son (Who clearly takes after his father) and decided that she should create another, one who would be more easy to manipulate the way she wanted. Unfortunately for her Odin came along and got in the way.
Jamie's shirt is cursed.Jamie gets injured a lot. And then Jareth borrows his shirt and this happens.