Plane Sailing
Astral Admin - Mwahahaha!
Aside from at-will powers, take a look at Daily powers which hang around for the duration of the encounter.
It would be great for things like Stinking Cloud - resounding thunder to increase the radius, Enlarge Spell because the -2 damage per die isn't so big an effect when you just have 1d10, and it gets around the poison resistance which undead and constructs have.
In fact I think almost all my paragon feats might go on energy admixtures!
