Well, this is something for me, of course!
Umbras have been a concept on which I based Cylonea on:
But I think it might be interesting to make a new mechanic.
At heart, Umbras are buffs. They always give bonuses of various types, and their core mechanic though is that a creature fighting with an Umbra is protected by it.
So an Umbra Enchanter, to me, is basically a leader that in buffing his/her allies, also protects them with an Umbra.
The Paragon Path could give bonuses to basically all powers that give bonuses to allies, with something like.
"Allies that receive attack, damage, or movement bonuses or actions from you also have Resist All 10 until the start of their next turn."
It's quite powerful, I know, requires more thought.
Thye idea was that by taking this Paragon Path, all your ally-buffing powers receive a new mechanic, on which the PP builds upon. Or maybe the 11th level power could be the new mechanic, and other buffing powers you have can interact with that.
An alternative could be that of making it a pseudo-pet PP. You have your pet umbra that acts a bit like a Shaman's Spirit, and allies can literally enter it and receive bonuses depending on the type of animal umbra (maybe decided after long rest). The cool thing here is that you could make it an Encounter power, the summoning. And say that if the ally using it is damaged, the Umbra is detroyed. This way, careful usage is suggested, since you can't summon a second Umbra in a battle. And at the same time, if used well, the benefits could be encounter-long.
Yes, I think this could be really cool. The problem could be the interaction with the Shaman's Spirit. We either make this a Shaman-only PP (bad), or we add a specific bonus for Shamans only, so as to say "This PP works best for Shamans, but other classes could benefit too".