There is a decent chance that this "Mystery Trial" will be Gilgamesh Extreme. Since Greg is still hanging around, and is obviously unhappy at us for defeating him way back in the first case, he'll probably send us a challenge to face him at his full power (IE, all 8 arms) at some point during or after the Hildibrand questline.

At least, that's my hope anyway. And I also hope he drops joke weapons, like Excalipoor, or Nragarok!