Atm one of the closest thing to open world dungeon is O'Ghomoro Mines for the Kobold beast tribe quests.
Can do it solo. Incentive to do as party for faster progress.
Not that complex to go around but it 'feels' like it and gotta double check map because of the side rooms.
It's like a mini-mini dungeon with multiple objectives (Kobold beast tribe quests) which can be tackled in closest order.
While doing it, joining in the FATEs is nice too.
Its a good base for further expansion for depth... But totally lacking in rewards or reason to stay long for serious 'dungeon crawl' or 'grind in a dungeon'.
(Anyways because its intended to grind in instances not open field).
- Field mob loot drops are... what did they drop again? T_T wasn't significant to remember and not enough.
- Not enough particular NMs or higher tier enemies to be fought in 'random encounters' (besides that big flying mark that spawns in the deepest room).
Could go on but for what it is, it felt to closest to typical dungeon zone and was fun too delve into it to carry out the Kobold's quests solo and as party.
If it could be further expanded, it would be worth while to visit more often other than daily Kobold quests.