The Trio!

[Nagash]

As you already have one shamy, I'd go shamy+monk+druid. Reason:
- Shaman: he's already there
- Monk: insane duo with shammy and I can't think of a single encounter a 3 persons group would be interested in that the monk couldn't tank
- Druid: a lot of utility (DS, snare, teleport, charm, buff...) + extra heal in case the shaman struggle + DPS through DoTs or DD
The only thing the group would miss is clarity, the druid will be the one sufferingg the most but hey, wouldn't that be the case with any group not including a bard or a chanty

Alternatively, if one of you is interested in carpal syndrom, you could replace the druid by a bard. Loose teleport but gain a lot of extra utility (super run speed, regen, mez, lull, charm...)