As of this moment in time, I am planning on rolling up a Druid and my wife will be creating a Wizard (she prefers killing to healing). The Dynamic Duo status for this particular pairing of classes has to do with Wandering Stones and Gateways. While I will be turning over every rock, log and leaf in a zone, getting a group with limited time into action sooner via ports is a wonderful ability to possess (x2). Healing and nuking are always 'two great tastes that taste great together'.
also rans:
If the undead are to be found everywhere, then Paladink and Cleric are in the running as a backup plan. Kinda hard to go wrong with tank/heals.
A distant third could be DL and Shammy. Might be oil and water, then again it might be set the oil on fire and watch the world burn!
Thunderleg
If it is anythin like EQ, playing an Enchanter will get you group invites before you are even fully loaded into the game.
You can pretty much pair anything with this to leach off their greatness.
"Enchanter will get you group invites..."
...IF you're a fantastic Enchanter. If you aren't...
Tank/Heals is almost always functional, no matter the game, and one or both are always needed for groups. Considering Pantheon groups can potentially go 2 tanks/2 heals, this is basically never going to be a liability.
Heal/DPS is also decent, since you're rarely going to find a group needing a DPS and no healers, and generally if a group needs a healer, they're going to be acomodating/have a slot they can put the DPS if they come as a "package deal".
Tank/Control or Heal/Control will also be vable options (Control being Bard, Enchanter...possibly Necro whenever it comes out..?)
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Though I'm a healer main by nature, I kind of feel like Monk may end up being the sleeper class. An off-tank that can deal damage would be useful for groups wanting to do a 1 tank/1 heal/1 control/3 damage to try to farm quickly and burn enemies. And with Monk having off-tanking, it adds the layer of safety of having a "second" tank, while still putting out respectable damage that a full on second tank could not.
I feel like Paladin (due to off-healing potential) + Healer (probs Cleric, but others might work) + Enchanter/Bard + Monk + 2 high output, low downtime DPS will probably be the "optimal" ginding/farming group composition. Such a group would have 1 and a half (Monk) tanks for that extra safety buffer, 1 and a half (Paladin) healers for if things go a little crazy, 1 control to keep things FROM going too crazy, and 3 dps (one of which is said safety off-tank Monk) for killing things quickly. Monk/Ranger are also likely going to be good puller classes, meaning having at least one in the party would be good for those reasons.