I think primarily debuff. BRD, COR, and to an extent RDM and WHM already serve the purpose of buffing your own party. I say RDM and WHM to an extent because RDM has a very wide range in abilities, and WHM does a lot of healing work.
I could definitly see a major debuffer job, like, being able to solo mobs purely by DoT in a reasonable amount of time.
today i walk in tringi cnayon and i want kill cameles for mad gill and i see a camle EATING ANOTHER ONE!!?!??! why am monster eat one other monster ??!?!?! i am 9 level ^_^ so i go help the good camel to win and i cannot why also when i want make gill this cannot be done ???!?!?!