Methos is my favourite, and am perfectly happy with him being the only Highlander character present. I'm also equally happy with any assortment of Highlander characters along with him.
Het is great, and a little f/f wouldn't go amiss, but m/m doesn't do a thing for me. Pairings, I don't really have an OTP, but don't mind Methos/Amanda, or Methos/Rebecca, and Methos/OFC would suit me just fine, or some crossover pairing. But would prefer no Cassandra or Alexa. If you do go the pairing route, I'm open to anything from PG fade to black to more explicit R rated fic. That being said, gen is perfectly fine with me, and I'm a big fan of friendship.
I love crossovers, and alternate universes, but a straight Highlander fic would be just as great. Highlander fic set in another universe works too, for example, a purely Highlander story set in the Star Trek future, but no Star Trek characters, that sort of thing. I enjoy stories where our Immies have to adapt to changing times - and that can be the future or the past.