From a layman's perspective on the concept of the animations I've narrowed it down to four stages of customization necessary on a basic level.
First the animation itself. How the character actually moves, and does he have to load a weapon first. If it's truly customizable there would have to be at least 3, and preferably 5 animations per every power-set already existing. If you're using an existing animation already it's not really a custom power.
Secondly the Aura around your character at the time of the animation. This is easy enough to do as Auras already exist. How long will the Aura stay with your character after the attack has completed? Contingent upon how the Devs built Auras it might be difficult to stack custom ones on top of each other.
Third and probably the most difficult are the Particle Effects on Explosive, Ranged, and to a lesser extent Melee powers. This is presumably the hardest to customize based on how they're originally set up. Take Energy Blast as a good example. There is a separate jpeg for blue, red, and green ones. There isn't a master jpeg where a color is applied after being loaded. (As costume pieces are.) This would have to be entirely reworked or a massive upload for the accommodating information.
Finally, the fourth layer would be an archive of various sounds. Obviously there has to be some kind of noise for your power. Thus new ones would have to be created.
Based on the amount of work required for such an endeavor it's likely that CoH and CoV will never see custom powers. There is a silver lining to all this though. I'm certain that in a new game (CoX2) it will exist. Something where the devs can build it from the ground up with this in mind from the beginning.
