The Grand Opening date has been released! Starting September 18, The Fairytale hall will be open. Sitting in Fantasyland where Snow White’s Scary Adventures was, the Hall will be the main meet and greet spot for princesses outside of meals at the Magic Kingdom. As it stands, there will be 2 meet and greets, one for Cinderella and one for Rapunzel. Both will also have a rotating princess greeting with them.
As with everything else, this is somewhat subject to change and Disney could swap things around a bit. Currently as far as I know, Merida will continue meeting over in the Fairytale Gardens and Ariel will continue to meet in her Grotto by Journey of the Little Mermaid.
Will post more as they release specifics.
The Disney Parks Blog has added a post confirming all this adding that it will work with FastPass and FastPass+ as well as the below video by Disney Imagineering giving a sneak peek.