We have a dark-themed tank (DRK), caster (BLM) and melee dps (Reaper), so I'm guessing a healer or ranged dps would fit the bill.

For healer, I could picture a mage that uses drain/aspir themed spells that absorbs HP/MP from enemies, and then redistributes it to their party members. Their hefty heals could have an HP cost, and they could regain it back with their DPS spells. Similar to a vampire, in a way, but more support focused. It's difficult to think of a weapon, but maybe a glove or gauntlet with claws (similar to the Ascians' gloves) that has a hole in the middle for their drain spells.

For ranged dps, the edgiest thing I can think of is a Van Helsing lookalike with a crossbow. Maybe give them a pet crow or something that attacks alongside you (but then again, that wouldn't be too different from SMN or MCH's Automaton).