It looks as though the English translation of Mirror Sword and Shadow Prince will feature the cover art of Magatama no Sekai, which was released in Japan last December.

 
Left: Magatama no Sekai
Right: Prospective look of Mirror Sword and Shadow Prince

Unsure as of yet if this will be the final design to hit stores. (Amazon.com still sports a ‘Cover To Be Unveiled’ as of this posting.)