[Athelos]

When Morrowind's mods started to bloom, a person released a mod that allowed the player to join all three houses (Hllalu, etc etc). I am curious though, is there a similar mod for Oblivion that allows the player to embark upon the Dark Brotherhood, Thieve's Guild and Knights of the Nine quest lines without repercussions?

If there isn't, it should be done.

[LFact]

You can join all available guilds at once in Oblivion(as long as you don't have bounty...). However, to wear Crusader's relics(from KoTN), you'll have to clear your infamy, and it is done by doing pilgrimage.