No idea. With the last 3 months all being a recap of the gen 1 and 2 legendaries it can be anyone's guess. I doubt the Regis would come up till after regigigas comes to raids. The lati twins are being re released now so if it was a legendary I would say kyogre groudon or mewtoo.
To be safe I wouldn't get my last stamp till I was ready to open the breakthrough