The Psion_Nomad specialty is also a pretty good choice for staying back from the fight, especially "Dimension Swap" to help your teammates position themselves more efficientlyor react to unforeseen tactical situations. ( combine that with 'Wallwalker' and/or 'Speed of Thought' and you can do some really wicked things ). Although Nomads become really fun mistly at higher levels.
If the campaign is going do be reasonably free of constructs, oozes and undeads, consider a telepath (later perhaps switching to Thrall Herd ) for some mindbending fun - and nuke some with the standard powers if the opposition is strong willed. "Empathic Transfer" and "Crisis of Breath" (both 3rd ) may be considered borderline broken and drive your GM up the walls, so use with caution.
A Shaper is a nice variant of the classic conjurer but its constructs lack a touch of flexibility IMHO