These aren't just ordinary recommends--these are the best of the best, the fanfic that simply Must Be Read. Take my advice and read these stories. You won't be disappointed.
"Incubus" by Tanja Kinkel
Summary: In the wake of Archangel, Methos and Cassandra form an uneasy alliance trying to find out exactly what's happened.
"Changed Utterly" by Janeen Grohsmeyer
Why I chose it: First of all, it's incredibly well-written. Every line, every characterization is dead-on. The best fanfic changes our perceptions of people and events, and this story converted me to Cassandra fandom, to appreciating the depths and possibilities inherent in her character. Secondly, it delves deep into the fascinating territory of Methos and Cassandra and who they are to each other. Finally, the two stories that come after this one are just as riveting. Read and enjoy.
Summary: Duncan comes back to the Highlands, mourning Richie.
"Heat goes to Cold" by Amand-r
Why I chose it: This is sublime fanfiction. Duncan's voice tells the story, and you become deeply involved in his emotions as he reflects on his life, especially the part taken up by Richie Ryan. History and mythology are interwoven into the narrative, and you'll find yourself wrapped up completely in the story. "Compelling" is the right term to use.
Summary: A day in the life of Methos. Nope, just that.
"Even Better" by Melina Clark.
Why I chose it: Because it's simply the best. Amand-r's narrative, in second-person, gives you an intimate glimpse into Methos' thought processes, his emotions, and the reasons for his actions. Nothing happens . . . and everything happens. I can't describe it. Go read it for yourself.
Summary: After taking her first head, Michelle Webster (the newbie F'immie from "Rite of Passage") goes to find her teacher, Amanda.
"Black Sand" by Jeanne Rose.
Why I chose it: First of all, anything that could make me actually like Michelle Webster is worth noting. Secondly, it gives a good account of what a new Immortal might just go through in their first few years "out there." It's very real. You understand what a struggle the Immortal life is, but it's also hopeful. You begin to understand how they survive. Thirdly, the interaction between the maturing Michelle and the grieving Duncan in this story is deeply touching. Give it a chance.
Summary: On the island of Santorini, after Bordeaux, Methos attempts to come to terms with his past. Haunting him are Alexa, in his memories, and Cassandra, in the flesh.
Why I chose it: Let's see . . . it's beautifully written, so much so that you can feel the heat of the sun, hear the ocean, and walk around the island with Methos, it's a story of redemption and forgiveness, which is what Highlander was all about, the characters are beautifully drawn, especially Methos, so that you can understand why he chooses to undergo the harsh penance Cassandra imposes on him, and you don't even hold it against her, etc. Simply put, you have to read this one.