Lightwand
Just a thought, because the Blackwand prestige power never really appealed to me for most characters, but how about an additional option that is essentially the Blackwand power recolored with a bright crystal that shoots a ray of light (deals Energy damage instead of Negative). Have it be origin/non-origin specific so that it excludes the other option for choosing Blackwand or Nemesis Staff. You could still only have three of the prestige attack powers, but it would present a thematic alternative to choosing a blast of darkness.
EMP Mace
A tech-alternative to the Ghost Slaying Axe / Sands of Mu pairing... the EMP Mace would use the same kind of damage formula (single target damage +bonus damage to specific enemy type). However, instead of dealing bonus damage to spirits and other (true) undead, the EMP Mace would deal its extra damage to mechanical beings. However, like the limitation on the Ghost Slaying Axe not affecting Vahzilok, maybe let the traditional Clockwork be immune to the effect because of their special psionic nature preventing the EMP from wreaking havoc on their minds. Like the Lightwand, selecting the EMP Mace would count against the number of prestige attacks you choose.
The list would be:
Blackwand (origin bonus/no origin bonus)
Lightwand (origin bonus/no origin bonus)
Nemesis Staff (origin bonus/no origin bonus)
EMP Mace
Ghost Slaying Axe
Sands of Mu
You could still only select of them, but only one of them with the origin bonus with increased damage.