Orsinium is just another storyline, it's pretty good but it's not going to change your world too much, but there is daily repeatable quests in the zone, there is also a solo arena dungeon as well that people like for the challenge and BiS weapons.
Imperial city is PVE in a PVP zone, you might find it fun, but it's not for me personally. There is also some dungeons that come with it though.
There's also another 2 dungeon pack, which, unless you really need to do those 2 dungeons, it's not really a big deal.
NOW
Thieves Guild and Dark Brotherhood? Those are game changing. Not only do they add new content to do, but they add game changing elements. If you want to be a dick ass thief that steals things, thieves guild adds a passive skill line that makes it better, reduced bounty timers, the ability to get out of an arrest without paying a bounty once a day, reduced guard detection range, the ability to use footpads to teleport to an outlaw refuge, also, and this is not part of the passive skill line but just in the world stuff, the ability to loot thieves guild caches in the wild all over the world, assassin's creed like hiding spots, special heist missions and other dick ass thievery dailies, and most importantly, clemency writs that let you erase bounty from your inventory without having to actually pay the bounty.
Now if you want to be a dick ass thief that stabs people in the dick, Dark Brotherhood is good too, not only does it add a passive skill line that reduces bounties from assaults and murders (and reduces aggro from enemies while you're on your mount, gives a speed boost after killing an innocent citizen, etc), but it introduces the blade of woe mechanic which allows you to instantly and silently 1 hit KO humanoids (you get 1/4 of normal xp, and I think the loot changes to stolen fencable items only instead of normal loot), also daily black sacrament quests that are like heists, and disguises that can make guards ignore you for 5 minutes