mage,
dark brotherhood,
thieves,
rebel/empire factions are already confirmed.. what else would you like to see?
I would like a return of a temple/religious order, I loved them in MW and DF.
vampire hunters guild.. there was a scant one in oblivion but never really reached its potential.
ranger squad.. I LOVE playing rangers.. I would love open ended ranger quests ie 'patrol mountain trails around throat of the world for the next two days'.. more open ended quests in general would be nice.
bandit faction... join a notorious group of outlaws who mug, pillage and steal for a living... set traps along the highways to ambush merchants. Think thieves guild but less organised and not afraid to kill if they have to.
necromancers.. no need to explain this one.
Working for one of the earls as a spy... think splinter cell with swords... infiltrate opposing holds.. rise the ranks of the house but secretly report to other earl.
bounty hunters... like the grey fox flyers in oblivion have wanted posters scattered around the land.. in taverns would be good like in FF12. Then you get a vague area and have to search. Having quest markers for this would svck... make the player ask around and do some investigation.
game hunters... hunt food for the local township.. where else does that venison in the general store come from? someone's got to get it.
bodyguarding/caravan guard jobs.
imperial legion... patrol highways, arrest people, bust khajiit drug rings, deal with the aftermath of tavern brawls.. and be able to say 'stop you've violated the law' to NPC's.
mafia like crime rings... drugs, smuggling, people trafficking, heists. etc...
these are just some of the ideas ive had..