I think priests having a class specific ally would be cool.
I was thinking this:

Guardian Spirit
3cc
2/2 All combat damage delt to Guardian Spirit is 0

Kinda treat this ally as a "ghost" where physical attacks would not
hurt it. It could only be affected by abilities. Basically would be
the same as Nightshade but with +1 attack and +1 cc except
the opposing hero would still defend causing weapon durability
loss.