I am looking to run a duo to play with friends that will progress with the expansions as well. My current thoughts are Monk/Chanter, Ranger/Chanter, Ranger/Cleric, Monk/shaman, warrior/shaman, ranger/shaman.
Beastlord / Druid will be my duo when available, but I'd go monk / shaman as a solid combo that would last for a long time.