Compare to any of the above cards. I don't see much of a reason to use yours to be honest. Expansive + affecting your own creatures.As you said, there are certain circumstances where you may want to kill your own creatures, but that's not very often. If I am so worried about a creature affecting me negatively, I'd rather put quint in my deck.
Right now, this doesn't seem to do much different than fire buckler really. It needs something that sets it apart...What if you did something like: "Shield: Reduces damage by 1. All non creatures gain +0|-1 each turn, all creatures gain +1|-1 each turn."
Interesting, now it's a mass acceleration for 5 turns. Does it disappear after 5 turns, or just turn into a regular shield?