Wall of Stone = earth
Wall of Fire = fire
Wall of Ice = (frozen) water
. . . so here's two options pulled out of thin air:
The Sorcerer forms an opaque
wall of noxious gas that covers
two squares. A figure moving
through this cloud must roll one
combat die for each of his Mind
points at once or on future turns.
Until a black shield is rolled, the
victim repeats all combat rolls,
accepting the worse result.
The Sorcerer creates powerful
winds that cover two squares.
This wall blocks all weapons but
doesn't stop spells. Any figure
trying to pass out of these winds
must first roll one combat die
for each of his Body Points.
Unless a black shield is rolled,
his turn immediately ends.
These ideas aren't playtested, but either could be nerfed by switching black shields to white for the cancelation roll. Either appears to fit as a Storm Master spell, though Wall of Wind also works as a Protection spell and Wall of Vapor could instead be a High Mage spell.