She's been mentioned by a few already, but Mizuki Nana is probably one of the few guests that would get me to buy plane tickets to anywhere in the continent on the spot. Even more so than anyone else in the short list just posted.
Would love to see some Vocaloid related guests and events too; I don't know what Crypton's plans are for their Vocaloids after Miku Expo, but I'm certain an appearance at next year's Otakon would do much to keep their upwards momentum going.