I'd rather have them just fix their already made questlines. That or add Warriors and Rangers guilds. Companions is more of a questline than anything else.

A rangers guld would definitely be great. I like this idea.

I agree that Ranger/Hunter/Pathfinder type faction would be nice. As it is there's very little to support that type of character in the game, if I'm not playing a Beguiler of sorts, I'm a Ranger.
And the fellow who said having a real Fighters Guild would be nice is right, the Companions aren't a Fighter's Guild. You do like two jobs, then we all know where it goes from there.

The companions are the fighters Guild of Skyrim, if you play the quest you know that they have a problem in the circle with Lycantrophy but this is something closed to the circle, the player is only allowed to take part on it because we have see too much already (Farkas transforming in front of a rookie q-), the only complain i have about this guild is that you are forced to embrace Lycantrophy to finish their main quest, you can cure yourself and the members of the circle (except Aela who don't want to be cured) after it's finished, but would be better if you are given the choice to refuse it since the start and still be able to complete the story line :3

Archers can still be mercenaries, so go to the companions. As for the Thieves Guild, I always do those quests regardless of role-play. My excuse is that, after having possessions removed in Helgen, you're guy is desperate for cash - haha.

I have an idea! Far in the North there is a giant wall made of ice! Rangers swear their life to defend the realm from the evils lurking on the other side of the wall. Wait, that's taken. Another: the decendants of long dead kings travel around the countryside, in which lives a race of small elven folk with hariy feet and big bellies, using their skill at fighting and survial to defend these peaceful food-loving little people. Damn, thats taken too.

But really, I think a mix of the Night's Watch and the Dúnedain would be cool, with an Elder Scroll twist to it. They could have once been a strong, respected force, but over the years they have fallen to little more than the shadow of their original glory. They swear no oaths of feality to any kings or emperors, and their sole goal is to protect their ancestral home from any threat that may present itself. They don't collectivly take a stand in polical strife, like the civil war, though a few do, allowing the player to continue the Civil War questline. They have no cities or castles, but they have gatherings (moot, war council, something like that) reguarlly to discuss their plans and at times, decide who leads them. These could be camps with about twenty or theirty rangers present, though I doubt that meny would be in the game, that moved from place to place, only stopping to make the council. Other than that, the Rangers traveled alone or in small groups.

They live of the land, are renowned archers and swordsmen, magic is scarce, with only a few healers in the whole group, they wear light armor, they could be kind of the Lawful Neutral kind of guild.