IMHO, the real problem with shields is that they scale with magicka so when you build for maximizing offense (glass cannon), you "accidentally" maximize shields as well and get a heavily tempered glass (cannon). I still think that both annulment and steadfast ward are ok, since they have a prerequisite (light armor and restoration staff). The only problem I see is a sorc in heavy armor able to shield stack.
The obvious solution would be to remove magicka scaling and replace it with something else.
But here is another suggestion: make conjured ward a pet skill (volatile familiar). This way, it would place some restrictions on builds (pet sorc), non pet sorcs would be in the same spot as other classes (i.e. light armor/restoration staff), in PvP others could counter it by killing the pet first, etc.