I dunno in a weird kinda way, Arcadia could work. In that WoTC seems to be going for the, "let your own creativity and imagination feel it in", so if they were to just blatantly rip off Arcadia it could work. Since... Well, each True Fae realm would be completely up to the imagination of the DM, given that it alters on a True Fae's whim. It would really be the Hedge that be most fleshed out.
But now... I am just daydreaming
![Stick out tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
In reality, I am hoping for perhaps them sliding some of the folklore and mythology of Fey into that part of the book. I have been tempted if I started a Fey-oriented one for example for it to be polite to give a Eladrin a glass of milk and biscuits/or bread if he enters his home. Since in folklore (especially Ireland) you leave bread and milk out for the Fey.
Overall I hope it deals with less the Feywild and more how the Feywild and Fey interact with the world. Lets find out what 4e versions of grooves, fey mounds, fey rings, etc. be like.