I had kind of a similar idea. I sort of envisioned that you could consume your shadow orbs and it gives you the use of unique abilities (kind of the way meta works for demo warlocks right now). I imagined the fabled shadowmend as one ability and perhaps shadow crash (you know, the spell those faceless ones use in Ahn'kahet) as another option. A third spell might be something like "Shadow Walk" where the priest could teleport to the target location by travelling through shadows (sort of like the "half men" in the Wheel of Time series).

If they were to work this way, however, I don't think the orbs should be gained through RNG. They would need to recharge slowly so they can be used as an "oh shit" on demand but not too often. This would also allow the shadow priest to choose when he or she used the abilities. Also, since these abilities would all be differant in nature, instead of a %damage increase with mastery stat, we would gain an increased rate of orb regeneration. The only problem I have with this sort of thing would be that it seems to me to be a sort of mash-up between runic power and warlock soulshards as they will appear in cata.

I suppose another way this could work is that the number of orbs there are would determine the magnitude of the spell in question:

Shadow Walk - The priest travels through the shadows to a target location with in 15/20/25 yards

Shadowmend - Instantly heal the target with shadow energy for 5/10/15% of maximum HP.

Shadow Crash - The priest launches a projectile of extremely condensed shadow that deals massive area of effect damage on a target location, dependant on the amount of shadow orbs consumed.

Oh man... wtb edit button; I just had an awesome idea about that shadow walk. Instead of a clone "blink" ability, it could be something new and unique:

Shadow Walking - The caster will place a path of shadows on the ground that is 15/25/35 yards in length that lasts for 25/30/35 seconds. The priest can "submerge" in to the path and will in it, gains 50% movement speed and cannot be targeted, but the priest cannot use any abilities or items while submerged. The priest can enter and exit this path for as long the path exists.