The relevant text would seem to open up the poor character to just about any physical change the DM's twisted mind can come up with.
Most of the physical changes listed in the RL rulebook for failed Powers Checks would be particularly appropriate, or "the usual": enlarged eyeteeth (fangs), scabby or barklike skin, enlarged fingernails (claws), catlike eyes, change in posture, unusual hair growth (length, location, abundance), size change in one limb (shrink or grow), that sort of thing. Nothing that gives the character a bonus or extra power, but nothing that gives the character a penalty or disability either.
Also, if playing with the 3E Ravenloft book, don't forget to gleefully increase the geomancer character's Outcast Rating, making it *much* harder for the character, and anyone associated with the character, to get along with the locals.
On the subject of Powers Checks, it might be fitting to apply the 1%-per-spell-level check for *any* spell the geomancer casts, doubled or tripled for the "naughty" spells specified in the RL rules (all Necromancy spells, etc.). And if the geomancer should cast a "naughty" spell for a truly selfish or Evil purpose, well then perhaps the road to Act of Ultimate Darkness is a little more tempting for a spellcaster linked intimately to the land in Ravenloft....
Hope the character in question is the morally upstanding sort. Heh. If you're in need of further ideas, you might try the message boards at the official Ravenloft d20 Web site:
www.kargatane.com.