I can only think of about two fortune tellers to cosplay as this upcoming year.
Theresa from Fable II
Hitomi form Escaflowne
I need a third, badly. And I don't even really want to dress as Hitomi, she creeps me out.
It has to be a fortune teller? Then I'm going to shamelessly plug my own gathering and recommend Anya Cocolova from Mahou Sensei Negima, and not just for a plug, but because that's the first fortune teller that comes to mind. Tho' it would be easier to suggest some cosplays if we knew what series you follow.
If it's just fortune tellers then it is still a rather sizable list.
Baba - Dragonball
Sarina - Yu-gi-oh GX
Rouge - Power Stone
Miyu Kuroi - Megaman BNS
Madame Taki - Mermaid Melody Pichi Pichi Pitch
etc.
Do you have any specified preference to who you are looking for?
I suggest Yūko Ichihara the Space time witch of XXXHolic and Tsubasa: Reservoir Chronicle