I was thinking that although I'm not going to ask for huge things like guilds with quest lines, I think that smaller less quest formed guilds could be a part of elder scrolls games.
Hunters Guild: Once joined, an individual has access to cheaper hunting tools such as bow and arrows and hunting knives, as well as poisons. It could also have a ranking system where you can sell pelts, bones, and similar things which would increase in price as you increase in rank. Thieves guilt oblivion's filler missions of gaining certain amount of coins by stolen goods could be used in a similar way. Joining could gain you a special set of hunter's clothes and boots for sneaking, a hood and even life detection spells. Maybe a board of requested animal parts (similar to GTA San Andreas car lists).
Martial Arts Club: Regular attendance for skills to rise quicker, availability of learning many fighting skills and rising ranks like the belt system of japan. Could offer bonuses like practice wooden sword which can't kill but can knock out.
Mensa: For players who're powerfully magic to find cheaper but powerful magical augmentation tools as well as maps and tools that would generally help magic players.
What would you like to see in this concept, if it was to ever be implemented?