Sadly I don't think he will appear as an Elder Primal after seeing Siren (who appeared in concept art at the same time as the Fenrir pic) get relegated to Pharos Sirius. If that's any indication as to where XIV is heading, then the people saying Fenrir will be in Amdapor Ruins will probably be correct. Lambs of Dalamud in Tamtara Deepcroft did have a "Dalamud"-esque magic ball going on. Fenrir would fit the role as "Menphina's Hound," which the moon itself is associated with Menphina and Dalamud (pre-descent) was described as Her Hound and the red tint was the Hound bathing in the blood of a Vile Fiend. So short answer, probably Amdapor Ruins and not playing an iconic role like Odin.