Druid is great if you want to keep a healer. They can do a bit of dps, and allow you to go into older raid content, and provide far superior group healing (the shortcoming of mercs).
Bard is great to boost your dps, but a bit redundant in this setup. If you were tanking and raid geared it might be worth it, but I'd still drop the enc for something else if you go w/ bard.
Ranger is pretty good. Solid sustained dps, and slacking on nukes doens't drop the dps *that* much since atm their nukes are pretty tiny (I do less than 2k nuke damage on a burn on mine, when I'm burning for 28k). If it's on a separate machine, you can use an autofire program w/ a good burn key to get very solid burn dps there. It can also serve as a solid backup tank to the merc, too. Hell, pretty similar tanking to the merc actually. They still take a decent bit of work to do good dps on, but it's not too crazy.