When you say DRK tanked in XI that was /nin and not FTW.. no DRK/DD tanked anything challenge worthy in XI. I also differentiate between being "able" to do something and doing something because it's a "winning" strategy. If you sacrifice a ton of STR and DEF to ATT abilities/gear to tank on DRK that's just a D rate tank.

1)Meaningful Situational: PLD+/- NIN(AoE=X), MNK(counter/weaker mob, salvage, non AOE)
2) Weaker mob/mostly situational: War, SAM/Seigan, DD/nin (ie: DRK)
3)Very situational/solo/ messing around: Rdm/nin, Smn/Bst pets

I think FF series wise GLD > pld+Drk would make the most sense since they are basically inverses, (equip sword> switch Scythe/GS @job change) You'd need a significant DPS increase to make this work, and XIV GLD abilities wouldn't make sense on DRK, but would have good survive-ability.

Ability wise MRD> AXE would make sense for DRK to use as well, plus the abilities would match up better with DD and could still wear heavy armor. So I think for XIV MRD> DRK would make the most sense.