I think glamours, glamor prisms, materia, and rare gathering mats would be fine. The key part is they have to be both random enough to foil people purposely farming them, and common enough that average players will find at least a couple without really trying.

Plus, I would hope that all items obtainable via this method, (included potentially mounts) would be tradeable/sell-able on the Market Board. Else, if there was anything exclusive, it'd be the worst RNG frustrating grindfest in existence. I don't think anyone wants ANOTHER Time Lost Proto Drake.... Or Vitrious Stone Drake... Or Time Lost Hippogrif... WoW had a LOT of these.