I appreciate all of the suggestions! I'd still like more as I haven't yet decided.
My worries thus far are that we still only have two equipment slots (and I really love the Druid/Warden combination) so it seems like it may be a hassle to have to keep track of my gear. I'm looking into classes now that use the same types of gear as either a warden or druid so that I don't have to swap out too much.
I like the Druid/Warden as a healer. I'm not in the end-game yet, but have been able to keep any group I've teamed with up for the most part. So, I'm not technically looking to replace this healer combination, but rather having the chance to assume a different role. Either as DPS/crowd-control.
I'm leaning towards the scout at the moment, but the mage is causing me the trouble. Both druid and mage combinations seem to fare very well but I haven't heard much about mage/warden or warden/mage for when I want to try something even different!
If it changes anyone's opinion, the character I will be playing this third class with often is a knight. He expects to go knight/warrior.