I'm thinking we should pinch some of a ghost's Special Abilities, namely Malevolence, Frightful Moan and Horrific Appearance.
We could just make it a sub-template of a ghost, but I'd rather make it a standard monster for simplicity's sake.
Apart from that, I'm thinking an "Eldritch Glow" power that functions like faerie fire, so they can become visible and terrify victims even if its pitch black.
I'd like to give them a cut-down Telekinesis equivalent to mage hand - maybe by touch only? - and the normal Rejuvenation.
Give them half a ghost's racial bonus (i.e. +4) on Hide, Listen, Search and Spot? Give them a hefty racial bonus on Intimidation? (+8? or +12?)
That should be a good start.
		
		
	 
Blast, I meant to put Manifestation, not Malevolence in the above list.
Anyhow, here are writeups for the other Ghost-based Powers I fancy for them. Any ideas on how to overcome their Rejuvenation?:
Eldritch Glow (Su): A galley beggar can surround itself with an eerie light with the same effects as the 
faerie fire  spell. This light allows the galley beggar to use its frightful  appearance power in conditions of darkness. Galley beggars can create or  extinguish their eldritch glow as a free action.
Ghostly Hand (Su): A manifested galley beggar can telekinetically move small objects as by the 
mage hand spell, except its ghostly hand has a range of touch.
Manifestation (Su): A galley beggar spends most of its time on  the Ethereal Plane were, as an ethereal creature, it cannot affect or be  affected by anything in the material world. When a galley beggar  manifests, it partly enters the Material Plane and becomes visible but  incorporeal on the Material Plane. Galley beggars can not manifest in  daylight (either natural or from the 
daylight spell) or within the area of a 
consecrate, 
hallow, 
magic circle or similar apotropaic spell - they have to manifest during the night, underground or in other areas of darkness.
A manifested galley beggar can be harmed only by other incorporeal  creatures, magic weapons, or spells, with a 50% chance to ignore any  damage from a corporeal source. A manifested galley beggar can pass  through solid objects at will. A manifested galley beggar remains  partially on the Ethereal Plane, and can be attacked by opponents on  either the Material Plane or the Ethereal Plane.
A galley beggar has two home planes, the Material Plane and the Ethereal  Plane. It is not considered extraplanar when on either of these planes.
Rejuvenation (Su): It's difficult to destroy a galley beggar through simple combat: The "destroyed" spirit will often restore itself in 2d4 days. Even the most powerful spells are usually only temporary solutions. A galley beggar that would otherwise be destroyed returns to its old haunts with a successful level check (1d20 + galley beggar's HD) against DC 13. As a rule, the only way to get rid of a galley beggar for sure is to 
X.
Turn Resistance (Ex): A galley beggar has +2 turn resistance. Should a manifested galley beggar be successfully turned it is forced to stop manifesting, remaining ethereal for the duration of the turning.