In addition to adding the Thieves Guild and Dark Brotherhood post-launch, it would be awesome if the developers added…
Hunters Guild
Basically, this NPC Guild would lead you on a series of quests to hunt rare and unique beasts. Imagine it takes an hour to hunt down and kill a rare animal. In addition to adding hundreds of hours of new content, a new Hunter Skill Line would be added. This skill line would allow access to active abilities such as Animal Companion (e.g. summon wolf), Tracking Animals, Tracking Humanoids, Hunting Trap, etc.
Archaeology Guild
Basically, this NPC guild would lead you on a series of quests to explore numerous excavation sites scattered throughout Tamriel. Players must travel around the world uncovering ancient artifacts, many of which have interesting tidbits of lore. The quests are rather difficult, requiring players to solve numerous puzzles and not relying on quest markers. Also a new Archaeology Skill Line would be added as well. This skill line could have passive abilities such as Treasure Sense (slightly increases the amount of gold dropped by monsters) and Magic Sense (slightly boosts the odds of finding higher quality items).
Thoughts?