How about we add the opposite of a pin field? Instead of having it push in on a spot on all 4 sides, have it push out on all 4 sides, making creeper unable to reach that subcell?
I Do not see any way to reproduce this kind of effect with any other set of fields, So I want this.
Finer note: pin fields do not "push" or "pull" as the other fields do. They "hold" any amount of creeper. Creeper may flow into them or it may not, but the field does not in any way pull creeper toward it. It just doesn't let it out, either.