A horde mode of some kind. Sadly, I stopped playing 1.0 before they let monsters in the main cities on repeat to experience it. I'm going to get crazy here, but one of the new main cities should have random attacks like how XI did Besieged and Campaign. If you lost, you have to rescue vital NPCs from captivity from the enemy stronghold. Said stronghold would be an open world dungeon, flying is disabled and enemies are tougher (like having B, A, S, SS Rank monsters). Only problem is this might be bad for low pop data centers, so they'd have to modify the strength of the stronghold depending on who's in the zone.

You know how Gold Saucer in FF7 had a battle arena? That would make for an interesting attraction on XIV's Gold Saucer. It's solo content like MSQ duties, but with good rewards. The BLU masked carnivale was such a solid idea that other combat jobs could benefit.