I've always been slightly annoyed by the generic standard guilds of YES so I wanted to think of some new more interesting ones. Here are a few:

The archaeology guild. There's an Oblivion mod called the lost spires which introduces one and it would be interesting to see the concept expanded. They'd delve into ancient ruins and tunnels looking for historical artifacts, scrolls and the like.

The Valenwood naturalists' society. This guild would be focused primarily on alchemy, but it would also be concerned with other types of field research as well.

well in valenwood it would be called the "green pack rangers" or something XD

and if you pick herbs or chop wood in valenwood you get a minis in reputation with them

I would like a Ranger guild that sees you specifically hunting down the monster races and defending towns, cities and people from them.

Well, they'd be studying herbs and different chemical properties. Maybe even experimenting with infusing magic into different alchemy combinations.