Leslie Fish has been writing an engrossing and intricate post-Endgame fic called Sins of Omission for several years now. Last summer it got put up on HLFiction and since then two more parts have gone up.
Duncan, Methos, Connor, Cassandra, Joe and several interesting OCs feature throughout, with appearances by Amanda, Alex and John (from HL3). Slash, het, plot and kink.
It is definitely NC-17, for both Saxons & Violins :-). Plus, still a Work in Progress, though most of the chapters come to reasonable pausing points.
Part 27: Wormwood Dialogues is just up and is an excellent read.
For convenience sake (my own as much as anyone else' :-)) I have listed all the parts in order below:
Part 1:
Dark WindowPart 2:
AshesPart 3:
EmbersPart 4:
Invisible SwordsPart 5:
Pathetic MonstersPart 6:
For RemembrancePart 7:
War in the Next RoomPart 8:
The Long Grey ShorePart 9:
The Blacksmith's DaughterPart 10:
Hasten the DayPart 11:
More than TeacherPart 12:
Duelling GodsPart 13:
Winter SoulsPart 14:
Passing ShipsPart 15:
The Turning of the YearPart 16:
Hard LessonsPart 17:
Hogmanay DancePart 18:
Shadowed LegacyPart 19:
HermitagePart 20:
RefugePart 21:
Greek ChristmasPart 22:
Out of the StormPart 23:
Streams Beneath the GroundPart 24:
Widening CirclesPart 25:
Bird of PassagePart 26:
Personal FoulPart 27:
Wormwood DialoguesPart 28:
Sea-Change