I'm not sure if you've read or rec'd these, but in case you haven't:
In the category of super-duper epics, similar to the snape-is-harry's-father but actually in the genre of snape-is-not-biologically-related-but-adopts-harry:
A Year Like None Other, by Aspeninthesunlight, complete with over 760,000 words and one or two sequels: http://archive.skyehawke.com/story.php?no=5036
Tira Nog, Until Proven (adult!Harry/Severus slash) http://tiranog.southroad.com/harry_potter/Until_Proven_Part-I.htm
Sinick and ac1d6urn, The Snidget (adult!Harry/Severus slash) http://www.thebejeweledgreenbottle.com/2009%20Mixed%20Games/Fic%20Snitch/snitch-sinick-ac1d6urn.html#taxonomy
Lamardeuse, Defenders of the Real (Merlin/Arthur slash, 50K words, so not super-duper-epic) http://boxofmagic.talkoncorners.net/viewstory.php?sid=23&warning=5
Staraflur, A long way to go before we are truly divine (Merlin/Arthur, 45K words) http://boxofmagic.talkoncorners.net/viewstory.php?sid=27&warning=5
Shine, In Time of Trial (Merlin/Arthur slash, 37K words) http://boxofmagic.talkoncorners.net/viewstory.php?sid=29&warning=5
Astolat, The Crown of the Summer Court (Merlin/Arthur slash, only 24K but absolutely awesome) http://www.intimations.org/fanfic/merlin/The%20Crown%20of%20the%20Summer%20Court.html
Rosemary, The Damage Control Affair (Napoleon/Illya slash, only 32K but also absolutely awesome) http://tiranog.southroad.com/UNCLE/damage-control-affair.htm
no subject
I'm not sure if you've read or rec'd these, but in case you haven't:
In the category of super-duper epics, similar to the snape-is-harry's-father but actually in the genre of snape-is-not-biologically-related-but-adopts-harry:
A Year Like None Other, by Aspeninthesunlight, complete with over 760,000 words and one or two sequels:
http://archive.skyehawke.com/story.php?no=5036
Other epics:
Alex, Convergence (Qui/Obi slash, 132K words)
http://www.masterapprentice.org/archive/c/convergence.html
Katbear, Master's Voice (Qui/Obi slash, 167K words)
http://www.masterapprentice.org/archive/m/mastersvoice.html
Tira Nog, Until Proven (adult!Harry/Severus slash)
http://tiranog.southroad.com/harry_potter/Until_Proven_Part-I.htm
Sinick and ac1d6urn, The Snidget (adult!Harry/Severus slash)
http://www.thebejeweledgreenbottle.com/2009%20Mixed%20Games/Fic%20Snitch/snitch-sinick-ac1d6urn.html#taxonomy
Lamardeuse, Defenders of the Real (Merlin/Arthur slash, 50K words, so not super-duper-epic)
http://boxofmagic.talkoncorners.net/viewstory.php?sid=23&warning=5
Staraflur, A long way to go before we are truly divine (Merlin/Arthur, 45K words)
http://boxofmagic.talkoncorners.net/viewstory.php?sid=27&warning=5
Shine, In Time of Trial (Merlin/Arthur slash, 37K words)
http://boxofmagic.talkoncorners.net/viewstory.php?sid=29&warning=5
Suiane, Deluge (Merlin/Arthur slash, 51K words)
http://boxofmagic.talkoncorners.net/viewstory.php?sid=12&warning=4
Astolat, The Crown of the Summer Court (Merlin/Arthur slash, only 24K but absolutely awesome)
http://www.intimations.org/fanfic/merlin/The%20Crown%20of%20the%20Summer%20Court.html
Rosemary, The Damage Control Affair (Napoleon/Illya slash, only 32K but also absolutely awesome)
http://tiranog.southroad.com/UNCLE/damage-control-affair.htm