Well, stealth from the concealment pool is great for any character that's supposed to be a spy or ghostly, yet doesn't already have access to such a power.
Also, check out the costume change emotes, as many of them are really good - such as summoning a murder of crows, raising up your hand to call lightning, or maybe even chugging a potion. You should be able to trigger the command by switching into an identical costume in a different costume slot, if you didn't want to actually change your appearance.
Force of Will has mighty leap, which grants you a footstomp-like power, which can be pretty dramatic, and requires no enemies.
Presence has Invoke Panic, which is a PBAoE, (so doesn't require any enemies to use) - though having to take 1 other power to gain access to it is iffy, IMO.
Otherwise, check out the other emotes available...